Forever stamps are always going to cover the current first class postage charge. So given the way we keep having postal increases, they go up in value.
However, undenominated first class stamps do not change (increase) in value to the current first class rate. They cover only what they cost when they hit the market, in the cases of the earliest ones from 1975, just 10˘. So to mail a first class letter today you'd need seven of them and a 3˘ stamp. You may also need to explain things to a postal clerk who is unfamiliar with the USPS guide to them at
https://pe.usps.com/QSG_Archive/PDF...00/q604a.pdf.
Hope this answers your question.