Should be possible if you haven’t started using it. You'll probably have to get the old pass reimbursed (see refund conditions of your pass) and then buy a new one. Best is to contact customer service.

It will depend on where you plan to go-in quite a few country there is hardly any 1st cl. on trains and in some others its simply not worth the money at all. Also if you plan to use a lot of those Hi-Speed trains (notably ES,FR.IT) that require extra supplmts, these are also much higher for 1st cl.

If you always will not do for less as bisnis cl. in planes 1st cl may be worthwhile, other wise probably not.

If you cannot exchange-in SOME (that means not all) it MAY be possible to upgrade for a certain trip/sector, by payng the normal difference bewteen 2nd and 1st cl tickets.

Can I ask a supplementary question, please? 

If l have a FIRST CLASS global pass, is it possible to take a second class seat and pay only a second class reservation fee? 

Yes you can do that.

Many thanks for the confirmation