The licence validity period of pre-JAR-FCL PPLs is stated within the licence. Most are probably valid 'for the holder's lifetime' - except that the Ratings must be maintained as for a JAR-FCL licence.
So - maintaining validity is the same. However, the big difference is that you don't have to pay for the nonsensical 5-yearly re-issue fee in the same way as you do for a Eurocratic JAR-FCL PPL.
I tried to tell the DfT that this was unreasonable and would be expensive; however, they prefered to believe the CAA's blatantly dishonest Regulatory Impact Assessemnt which claimed that the JAR-FCL PPL(A) would be no more expensive to maintain than the old UK PPL(A) had been. An outrageous lie.....
If you have a UK PPL(A), you do not have to convert it to JAR-FCL PPL(A), nor should you EVER consider doing so!