transfer to the reserve
I don't know the mechanics of transferring from Regular to Reserve service but I will try to help with your pensions question.
AFPS 75 membership is not currently open to those serving in the Reserve. Excuse my cautious answer but historically those who left with an AFPS 75 immediate pension who later did an operational tour did accrue further AFPS 75 benefits. This change on 6 April 2005 when RFPS was introduced and those on FTRS/ADC commitments and Reserves on operational tours were pensionable under that scheme.
From 1 April 2015 there will be one pension scheme for the majority. AFPS 75/05 members who were age 45 or over on 1 April 2012 and RFPS members who were 50 or over on that date will remain in their current schemes but everyone else, the majority, will transfer to AFPS 15.
The introduction of AFPS 15 is good news for the Part Time Volunteer Reserve (PTVR) as it makes their service pensionable for the first time. It is in this scheme that your new device will build up. If you leave the Regulars as an AFPS 15 member and join the RTVR reserve within 5 years your service is taken as continuous. If the break is more than 5 years you will start to build up a separate entitlement.
I hope this helps.