Can attackers change transactions during a 51% attack?
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.