Elco's do deteriorate but "un-used" it takes a hell of a lot longer.
For instance I would change elco's after 10-15 years when they have been permanently on the grid (speaking from my own experience).
The exchange (say if you pick the same elco's) will result in a crispier "more alive" sound.
Mind you deterioration of an elco is a bit comparable with tubes.....you can change the item "any-time" it is a bit up to you and how "un-easy" you are with your sound.
I do not particularly "hunt" for NOS elco's since I believe that the newer ones are giving great sound as well. My personal favorites besides the unobtainable Blackgates are: Oscon, JJ, Elna and Jensen Silver Foil.
Maybe in your case the Jensen could be a lovely alternative.
The bigger Mallory was something like 20.000 uF/85V.

