1. The RFC is a recommendation, not a requirement. Not following it is not breaking compliance, it's breaking the recommendation. Please stop getting this mixed up, as it is confusing other users as well.
2. Changing your DNS Serial (be it a running serial from plesk or a date format that is as the RFC recommends) w/o an update, and w/o utilizing the Plesk system DOES cause problems on more than one level. Please be aware of this, as it can cause you additional problems with your DNS systems. Yes, you can get away with it, but you CAN run into a LOT of trouble if you do ONE thing wrong.
3. Users unfamiliar with zonefiles should not consider manually changing the dns serials to meet RFC recommendations. I'm sure the people at SW-Soft are working on offering an option to allow the Admin and/or Client to select which method they'd prefer to use. If you mess something up, they will charge you their hourly rate to fix it. It wont be cheap, you will regret it. You've been warned.
4. Serge, you can fix your secondary DNS Server to read the new DNS by clearing that zonefile completely and doing a restart. It'll force the zone to reload from scratch, and it'll pull the data from the serial, instead of the date format. Any future updates to the Plesk DNS will automatically fall down (if you have it listed in Plesk as a secondary), allowing automatic updates of your secondary servers to work properly.
5. Thank you for listening to my mini-rant. I'll vote +1 for the DNS Date Format, as it IS recommended in the RFC Documents, but until it's supported, I'll deal with the Serial...