Questions about getting new vinyl liner..

W e will be replacing our inground vinyl liner this spring.( after my wonderful Mc Guyvering to hold my liner up at the end of last summer!). Since the pool was here when we moved in , with fairly new liner ( lasted us almost 20yrs!) I am just curious as to how the pool company will measure the pool for the new liner? We have had water behind the liner for many years, which we drained off each season, and it worsened as the liner got older. From all I have learned on this site ( lots of help always!) I understand that as the liner gets older it looses its stretch etc. Also , I truly believe that the liner never actually fit the pool 100 % which lead to the water behind liner issue...because the measurements werent accurate possibly?
HOw do they do all the measurements properly with an old, stretched out liner in place?..or do they remove the liner first?
I am sooo looking forward to the new liner and bead track this summer....I have spent several years reviewing and adding to the stickys about liner pulls LOL