Replaced the Crystal?

As far as I know it truly depends.

For example if one is to change the UG polerouter date's famous plexi with trapezoidal window to a version with a wrong sort of lens or one without a lens at all the value is affected.

Then again if one is talking about a Hamilton with a unsigned crystal or plexi and that is replaced I do bot think there's much value drop.

All in all , disregarding special cases (e.g. the UG Polerouter Date) , the plexi/crystal is in my opinion the least of the factors affecting value/price. Although even in the Polerouter example I would still consider ot the least of the factor, just that it would affect it more than in other relatively similar period watches.

Canuck is right. In my experience it is a case by case (pun not intended) situation as far as value. I have bought watches where the value was enhanced because of crystal replacement. Omega watches should have original crystals whenever possible. This helps to avoid damage to the dial and from moisture intrusion from ill-fitting, non-original crystals.

A 55 year old watch isn’t likely to have a good looking original crystal. Maybe the watch was kept in a sock drawer it’s whole life but more than likely it’s been replaced or should be replaced. As mentioned above there are very few watches where this matters. Even watches where original crystals are important they can usually be found on eBay for a reasonable amount.