Is it possible to change to protocol, mine lots of blocks offline, and then change it back? Is it possible to make changes in Bitcoin protocol and then start to mine faster than 1 block in 10 minutes?

What happens when someone makes those changes and "mines offline" blockchain that is longer than current one. And after that changes protocol back to "normal" and appears back to online network?

Is it even possible?
Such an attack is not possible:

The protocol contains a rule how to calculate the difficulty to succeed in mining a new transaction block. This difficulty expresses in the form that a hash must fulfil (number of leading zeroes) in order for other network participants to accept it as valid.

There is no other way to arrive faster at a hash that fulfils the difficulty requirements than to try out as many as possible as fast as possible.

Now, you could easily reduce the difficulty requirement in your client's protocol and mine thousands of blocks every second, but these blocks would not be accepted by the other network participants. Firstly, of course, because you don't fulfill the difficulty and secondly, because the length of the blockchain is measured not in the number of blocks, but in the combined difficulty of the blocks.