Regulars who rejoin as a Reserve will see their pension abated. This applies to those on Full-time Reserve Service (FTRS) (AFPRB, 2018, p.20).
Further, Reservists are expected to leave the Service at age 55 but the Armed Forces Pension Scheme 15 does not pay out until age (AFPRB, 2018).
However, there is a practice that most contracts will be extended if required.
“One such barrier is the abatement of pension. This applies where Service personnel who receive a pension are then re-employed on a new contract anywhere in the Armed Forces, including Reserves. The pension in these circumstances is reduced to ensure that total income from pension and earnings does not exceed pre-retirement earnings.” (AFPRB, 2018, p.21).
AFPRB (Armed Forces’ Pay Review Body). (2018) Armed Forces’ Pay Review Body Forty-Seventh Report 2018. Available from World Wide Web: https://www.gov.uk/government/publications/armed-forces-pay-review-body-forty-seventh-report-2018. [Accessed: 22 January, 2019].