I presume that your licence will include B737 300 - 900 rating in which case there should be no problems. Obviously there will be operating differences but again not really a problem.
Just mention to the TRE doing your LPC that you need to complete paperwork for issue of an ATPL and he/she will gladly oblige. I believe the only difference is that a raw data ILS must be flown - but do check!
Once ATPL has been issued and you're checked to the line (which shouldn't take long for someone with 738 experience) your salary will be £45k.
Best of luck with your application.