Changing DNS serial number to be in the past
I haven't had to do this myself, but I bookmarked a solution (HOWTO Fix SOA RR serial numbers) from the author of the Pro DNS and BIND book in case I make this mistake myself.
If his number with starting with 2033 is greater then the YYYYMMDDXX standard then you can reset the value.
Here is an article that describes the procedure. Basically you have to exploit the fact that the serial number is a 32 bit integer and will wrap if you use larger values.
You can set the serial numbers to whatever you like. By default the secondary servers won't pull a zone transfer unless the number is higher, but you can command them to force a transfer and reload as long as you have direct access to them. Just set the serial number to what you like, then issue the re-transfer commands to the secondary servers so they will go fetch the new information despite the lower serial number.