I'm in the same boat as Texas. I moved to Canada as Landed Immigrant while doing AOS in US.
I've valid EAD but previous AP expired. Must I have new AP to return to US and work?
----EAD has nothing to do with return, EAD is only for work, you need AP to return if you have used EAD for work and your I-485 is pending, if you have still H1b visa and you are maintaining H1b status for work you can return on valid H1B
If not needed, will this cause any problem for GC (US) in future?
-------- If your AP is Expired while you are not in US you will not be allowed back in US with out valid AP, from your case it seems that your I-485 is pending and your AP has expired while you are out of US, if this is the case your I-485 will be abandoned and you can not enter US
If you are in US and your AP is expired then get new AP before you leave US.
Canada landed or Canada PR status does not give you any privilege to return to US with out AP if your AOS is pending
I hope Murcury6 had the same case his AOS was pending and he went to Canada and his AP expired while he was in Canada he was not allowed to enter US. you can confirm with him and INS gave clear guide lines in past