Just curious on *why* you need to increase it by more than 1000?
If you are trying to make it higher than some secondary server, then you should just "reset" the serial. Then the number that Plesk has should be authoritative.
To reset the serial, add 2147483647 to the current serial number (see
http://www.isc.org/sw/bind/arm94/Bv9ARM.ch08.html#id2592850).
Wait for the secondary server to update to the new serial number.
Now, change back to the original serial number in Plesk, and all secondaries should behave correctly.
In Plesk, the named record is in the psa/named/run-root/var/ directory, under the domain name. I've successfully (temporarily) changed the serial, restarted nameservice, waited for propagation, and then changed serial back and restarted nameservice again.
I assume that the SOA is also in the DB, but I've not had a problem with it being overwritten in the short time it takes for propagation.
Hope that helps someone.
Mark