Yes. Passengers exchange berths by mutual agreement on more or less every long-distance train in India, and it is treated as normal.
It helps to be precise about what a swap is, because it is often confused with transferring a ticket, which is a completely different thing and is restricted.
What a swap actually is
A swap is a private arrangement about where two people sleep. Nothing is transferred and nothing is edited.
Your ticket, your PNR, your name, your coach and your berth number all stay exactly as booked. The chart is unchanged. You and the other passenger simply occupy each other's berths for the journey.
This is why it is unremarkable to railway staff: from the records' point of view, nothing has happened at all.
What the TTE thinks
In practice, TTEs generally do not object to a consensual swap within the same class, and frequently will not notice one.
Where it is worth speaking up is if the TTE is checking tickets and finds you somewhere other than your allotted berth. Say plainly that you have exchanged with the passenger in the other berth, by mutual agreement. That is the whole explanation and it is usually the end of the matter.
Two things to keep in mind. Stay within the same class — swapping a sleeper berth for an AC one is not a swap, it is travelling in a class you have not paid for. And both people have to actually agree: a swap is a favour, not something you can insist on.
Why anyone would take your upper berth
People new to this assume nobody wants an upper berth. Regular travellers know better.
An upper berth means you can lie down at any hour instead of waiting for the side seats to be folded. Nobody sits on your bed during the day. Nobody walks past your feet. For a solo traveller who wants to sleep through the journey it is the better berth, and plenty of people pick it deliberately.
A lower berth suits someone who cannot climb comfortably, someone travelling with a small child, or someone who wants to look out of the window. These are different people with different needs — which is exactly why the trade works.
The only genuinely hard part
Finding the other person. Traditionally you board, look around your bay, and ask the five or six people within earshot. If none of them wants to trade, that is the end of it — even though somewhere on the same train there is very likely someone who does.
That is the entire problem BerthSwap exists to solve: post the berth you have and the one you want, and see everyone on your exact train and date who is looking for the opposite trade, before you board.
Common questions
Is it legal to swap berths with another passenger?
Yes. A mutual berth exchange between two passengers is a private arrangement about seating, not a transfer of tickets. Your ticket and PNR are unchanged, and TTEs generally treat consensual swaps within the same class as normal.
Do I need to inform the TTE if I swap berths?
You do not usually need to volunteer it, but say so plainly if asked during ticket checking — that you have exchanged berths with the other passenger by mutual agreement. That is normally the end of the matter.
Can I swap a sleeper berth for an AC berth?
No. Swaps should stay within the same class. Moving into a higher class you have not paid for is travelling without a valid ticket for that class, regardless of any agreement between passengers.
Railway rules change from time to time and a lot is left to the discretion of on-board staff. Treat this as a traveller's guide, not an official ruling — check with Indian Railways or IRCTC if something matters to your journey.