90% of my business is modifications to existing pedals.
I get asked to do a lot of crazy things to pedals. Some of them I do, but when asked to mod a vintage pedal there are lines that shouldn't be crossed. Other times the line is blurry and other times its clear as day what to do.
There are always things to think about when it comes to modding a pedal vintage or not.
Lets start with resell value.
Does modding a vintage pedal decrease the resell value?
In some cases it would decrease it. The exceptions to this would be was it modded by a famous modder at the time the pedal came out. Say you had a 1969 Vox wah modded by Dave Weyer. That would definitely be worth more than a stock 1969 wah since he is known for modding Hendrix pedals. In most vintage sales people want it bone stock with all original components. Especially in the Vintage Guitar market. Pedals are a little different but still for the most part carry this rule. You will always have vintage pedals with a cap, resistor, switch or jack that has been changed out due to a repair so Every situation can be a unique one. These changes of course will dictate the value of the item.
For instance modding a vintage Uni-Vibe or other rare $1800.00 plus pedal, seems crazy to do that when that pedal sounds so great as it is! Also why bring down the value of a classic like that. Its like taking a 65 Mustang convertible and making it a 4x4 with monster truck tires. (which I'm sure someone has done) Then there are Pedals where millions are made, like a a 70s Thomas Organ made Crybaby Wah pedal. I mod those all the time since they are not rare. People ask for Truebypass, and other options on them. If modded right it can be put back to stock with a little soldering if needed so its not like you are putting monster truck tires on that 65 Mustang Convertible.
The second thing to ask is, "Does this pedal suit my needs as is?"