The old UK R/BCPL still exists (if you already have one) and lasts for 10 years. The 5 year requirement was part of the fraudulent Regulatory Impact Assessment which the CAA pushed through in order to be aligned with JAR-FCL before it became mandatory. Despite many protests, they alleged that the lower initial issue costs, plus 5 yearly reissue, was cheaper than the old lifetime UK PPL issue. This was clearly utter rubbish - but the requirement was pushed through nonetheless.
Which is another reason for the lifetime NPPL. At present, the NPPL has a lot of faults mainly due to the CAA failing to amend the requirements as agreed with the NPPL Policy and Steering Committee. Hopefully these will be ironed out this Spring, but we're at the mercy of the glacially slow CAA parliamentary drafters..
There is no reason why you should EVER change a lifetime UK PPL(A) to a JAR-FCL PPL(A), nor any reason why the CAA should not add a MEP Class Rating to a UK PPL(A) or UK R/BCPL!