With non-flexible ticket the answer is (at least previously) not, I've been denied boarding for earlier flight with many free seats when holding ticket for the next one, all this as AY Plat. Only way of getting on a earlier flight was to pay the difference in fare class and pay for a flexible ticket. This has applied both to many economy fare classes but also for I class (biz-non-flex).

I have heard some rumours that they have/will change this in the future at least for certain fare classes (not only Y/B/H) and also status should play some role. I haven't had case recently so I cannot comment on 100% but I do not remember seeing anything in written about this.

If no one knows the answer for 100% I will ask this later from my Finnair contact who usually has the latest info in such cases.

Edited to add: the above naturally makes very little, if any, business sense as it's freeing up capasity on a later flight and I have no experience of such cases at HEL, where AY is still (at least now) doing the ground handling by themselves. My cases have all been outside HEL, maybe they have a bit more flexibility / ability to make logical decisions at their home base. I fully understand that el cheapo tickets cannot be changed for a later flight, that's very clear.