No it is not worth replacing the original choke, reverb driver, and OT with Mercuries if they are still in perfect working order. I mean Mercury is good iron, but also very expensive ( almost double or more in some cases vs eg Hammond) and not that good ( that much better proportional to price). Also usually you don’t get that investment back in terms of resale value. The originals are almost always worth more. If you need a replacement and want something flashier than Hammond, check Classic Tone first. Often not much more than a Hammond ( but in US so the shipping can kill it; still much cheaper than Mercury). They don’t have as many model-specific replacements as Mercury, but just about any Fender/Marshall/Vox they got. For my Sunn or other more obscure/ not as popular brands there are much fewer options other than Mercury which are super expensive due to huge.
That said, if anything is ‘upgraded’ the most value is the OT. The guy who sold this to you knew that and that’s why he lied ( I doubt it was an honest mistake). That sort of ‘error’ would keep me from buying.
For future you can tell PT because :
- proximity to rectifier tube and power cord
- lay down flat style ( one winding is up, vs 2 lobes left and right with core in middle). Usually this means PT but a few OT s are like this, and sometimes PTs aren’t, but most of the time.
- if one T is in the middle vs one side or another, that is the OT ( also proximity speaker jacks and power tubes).