Can attackers change transactions during a 51% attack? If in a 51% attack transactions can be reversed, isn't it possible that the chain with more hash rate also changes transactions (for example if x sent 5 btc to y, they would change it to x sent 500 btc to y) 
and then would confirm such malicious transactions more often than the corresponding good transactions that were confirmed on the honest chain.

I have this notion that transactions get n number of confirmations if after the block with that transaction was added, n more blocks were added.
No. A 51% attack cannot modify transactions or those transactions become invalid. Blocks do not dictate whether a transaction is valid. Transactions have their own validity rules and if a block includes an invalid transaction, then that block is invalid, regardless of the Proof of Work.

Changing a transaction's details as you suggest requires producing a new signature. Only the original creator of that transaction can do that. So attackers cannot just arbitrarily change people's transactions as they do not have the private keys necessary to create those signatures. Thus any modifications they make will result in invalid transactions.

All that an attacker could do is remove transactions from history and insert their own transactions. They could only replace their own transactions. The danger of a 51% attack is that the attackers can replace their own transactions paying someone else with a double spending transaction that pays themselves thereby undoing a transaction.