Hello! I hear, and see, a lot of people who, without any consideration, replace their fender amp speakers like their changing tires on a car. My question is... can you take a Fender Twin reverb amp, say for example, and put, one or two, 8 ohm, or what ever ohm. into an amp that is set up for say... 4 ohm? without damage to the amp? thanks.

The answer is NO! . The ohms have to match the amplifier. If the amp is set up with and impedance of 4 ohms and you put 2 speakers 4 ohms each wired in series you will burn it up because you are at 8 ohms. If you take those same two 4 ohm speakers and wire them in parallel the amp won't sound right because you are now at 2 ohms half of what the amp is set up for.
It is a mater of OHMS LAW

The current flow on parallel connections is increased but the impedance (ohms) is lowered because of multiple paths.

In the World of Amplifiers OHMS Laws is a rule to be followed.

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The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.

Thomas Jefferson

+1, you will damage amp if you don't stick to proper ohms/load.

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2012 American Vintage Telecaster 1958
09 Fender american deluxe strat.
97 Gibson les paul special P90's double-cut
65 Princeton reverb re-issue
2010 Guild Gad-F40e

You can't get around or change ohms. The other gents are right on. Good explanation.