I think a lot of people have a stamp-buying spree just before prices zoom up, to save a little bit. But no, the advice given above is quite right.
If a decimal stamp has a number value - say 19p, then its value is 19p.
If a decimal stamp says "1st" or "2nd" or "1st Large" or "2nd Large" then it still has first class or second class full value, even if the price of stamps has then risen after its issue.