I have couple of couple of two years old perennials in large pots. These need fresh soil as quite a bit of soil and nutrients have drained out. Problem is that it's not easy to repot these due to size and weight. So my question is:
Will it be sufficient if I just top up the the remaining soil with extra fresh quantity instead of repotting? Will it give fresh life to plants or just a waste of soil?
Anyone having experience please.

Definitely add fresh soil. Adding a topdressing of compost, a quart or so depending on the size of the pot will give the plants an extra boost and help them to thrive by giving them nutrients as well as acting as a mulch.

Depending on the plant species, adding additional grow medium (or compost) atop the existing grow medium could cause rot diseases of the stem, crown, roots, that's why the advice to be careful about planting too deep when planting out.
Al

Some potting soil is labeled as having fertilizer, other may not have any fertilizer according to label, just bear in mind.

The impression I get from potting media that advertise they contain fertilizer is much like the bread wrappers that say "made with 100% whole grain wheat flour". While it's true that the bread might be made with 100% whole grain wheat flour, they don't tell you that the 100% whole grain wheat flour only makes up a tiny percentage of the flour used to make the bread. Even if it only contained 1% wheat flour, it wouldn't be a lie. Most of the media I see has a few fertilizer prills just so they can make the claim it contains a starter charge of fertilizer, but I wouldn't bet on the amount it contains being sufficient, or that you can rely on not having to fertilize for whatever interval they asdvertise on the package. Caveat emptor!
Al

Most commercial potting soil mixes contain little or no mineral soil. They are mostly various organic materials. These materials break down over time. As this happens, the soil level goes down in your pots.
Yes, you can add fresh potting mix to the top. Just be careful not to bury the plant tops, which might then rot. It may be best to top off the containers early in Spring, before new growth starts. Depending on what you're growing, just be careful about burying enough of the plant to cause rot.
Because the soil in the bottom of the pot is where the roots are, and that is the more depleted soil, the plants would probably benefit from regular addition of small amounts of complete fertilizer. Again, this depends on what you're growing. Some plants need little or none, some can take lots.
Even with this attention, the plants will eventually start declining. This is generally from root congestion. I've also had tree roots grow up through the pot's drainage hole, choking out my flowers. This, however, is likely to take many years.
I then recruit some help to empty the pots and start over.
They just don't stay on autopilot forever, at least for me!

sudmon said: These need fresh soil as quite a bit of soil and nutrients have drained out.

There two different issues you are mentioning:
1) 'Loss of soil'
As in- the soil mass that holds the roots is leaking out the drainage holes, so the soil level in the pot is decreasing. Along with this, it's been mentioned our potting soils (unlike our garden soils) are mostly organic and continue to decay- and this can cause the soil level to drop.

2) 'Loss of nutrients'
As in- the soil structure doesn't contain enough nutrients for maximum plant growth. It may never have had the optimum nutrients, or it did at one time and those were used up by the plant and/or leached out with each watering.

Both can be happening at the same time, to be sure.
But you can treat each as a separate issue. In other words, I would add soil if it needs more soil (or repot,) but look at besides compost for nutrients in potted plants.

For #1- when the soil level is dropping, I top them off a bit, but eventually I repot, as the roots are nearer the bottom of the pot, and I prefer to have them getting into the fresh soil. Adding inches to the top of the soil doesn't generally stimulate additional root growth upwards. Others have mentioned crown rot, another problem.
Sometimes I cheat on the smaller pots...
Tip it on it's side, slide the rootball out, add soil to the pot, slide it back in.

For #2- There are lots of options to fertilize your container plants- even in 'stale' dirt. Liquid fertilizer, or fish emulsion, or sprinkling your favorite granular fertilizer or organic fertilizer onto the soil and mixing it into the top few inches. The package should tell you how much to use for granular or liquid fertilizer, based on pot size. I've used a stick to poke a deep hole into the root zone and then sprinkled some time - release fertilizer into it. Timed-release (like osmocote) needs to be buried to be effective. Do one or the other- time-release or another method, not both!

I also sprinkle rock dust/minerals a few times a year and then tease the top of the soil to mix it in without disturbing the roots. I use granular Azomite- because it's local and therefore cheaper to get for me. Greensand or granite dust might be available to you locally to add minerals to your 'mostly organic' potting soil -filled pots.

Does this give any more ideas?

I have had a similar problem with heavy pots. I usually put down an old tarp and gently roll the pot over on it. Using an old kitchen knife or machete run it around the edge to loosen the soil and roots. Using a curved tool or your hands grab the dirt and work it to pull the plant loose and out. Add a potting soil and about add about 1/3 of its volume with perlite. Put the soil in the bottom of the pot so when the plant is put back it will be at the correct height. Add a small amount of the proper fertilized and a root stimulator.