If you move to a new data directory you need to export your account data and re-import it again. That will copy over the salt used for the account age witness which determines your trade limit (1 month 0.0625 BTC, 2. month 0.125 BTC, afterwards 0.25 BTC).
Unfortunately I overlooked another important part when I implemented that feature: The signature pubKey is also used in the hash for the account age witness.
So to successfully transfer your account without starting with the 1 month limit again you need to copy over the signature key. You can do that only if you do not have any offers, trades or disputes open!
If that is the case shut down Bisq, go to the data directory of the old app and copy the sig.key.
The path is:
[PATH TO DATA DIR]/btc_mainnet/keys/sig.key
Add it to the new data directory (back up old first) and start Bisq again. Then the account age should be the same as you have seen in the old app if you select the payment method.
I will try to find a more convenient way to do that in future.