From Professional to General Service and back

[finsalscollons]

Good morning.

I have read that, if you start your UN experience in a job included in the General Service category, it is very difficult to move to a Professional (P) job. You get stuck in GS category.

In my case, I have worked as a P3 and I am interested in a GS vacancy. If I get this GS job, will this mean that I won't be able to get another P job in the future?

To say it with humor, is it true that "once a GS, always a GS"?

Any information would be greatly appreciated. Thank you in advance,