Metropoulos Forum

Is there a possibility to change the impedance from the standard 4 ohm to 8 ohm on my 1964 Fender tremolux (piggyback) to match with an 8 ohm cabinet? And if it's possible: will it alter the sound?

You would have to get a new ouput transformer. Older Fender OT's aren't selectable in any way.
But why bother? Fenders are very tolerant of an impedance mismatch one step up the ladder (such as your four and eight ohm case.) I ran my '68 Bassman into an eight ohm cab for years, and many others have done the same. Plug 'er in and play yer guitar.
No two ohm or sixteen ohm attempts for you, though.

Is it not impossible to blow up an OT by running into a higher impedance? I thought it was. I thought the only serious issue was running into less impedance of which only extreme cases would be serious in the real world even with Marshalls.

I'd hate to say it's impossible to blow one with a higher impedance cab than the OT is looking to see, but it would be unusual.
Suffice it to say that Fenders, however, are FAR more tolerant of a mismatch in impedance than many Marshalls. Using their extension jack even almost guarantees that impedances will be mismatched, and in the opposite direction; two typically four ohm Fender cabs would drop you to 2 ohms on a four ohm tranny and yet most are still kickin' to tell the tale. Marshalls I just don't screw with, set 'em for the right impedance period.