About pre-euro stamps and sticker stamps

Just wondering if old (that is pre-euro) stamps are still valid to use as postage on actual covers, postcards, ...

In France, Belgium etc. one can still find pre-euro stamps used as postage which makes for fine covers given the amount of stamps with low face value that have to be used.

So how about Greece?

The Pontus Lyre, 1975 Hellas n° 1333

This stamp is one of the very few mint pre-euro stamps I have in my small collection of greek stamps and is part of the Musical instrument series issued in 1975. (of which I only seem to have this one it seems)

Another item which popped up while sorting out and soaking recently arrived greek stamps.

I have a problem with some to get them off from paper and wondered if it would be sticker stamps. Up till now I didn't encounter any sticker stamps or at least none that would stick to the paper.

But I have real trouble with soaking these ones off :

Are these sticker stamps?

Any of my greek visitors might have the answer?

Thanks in advance.