I will need to raise the temp to 84-6 F for the Discus.

Why?

It's not "required". I had and _bred_ my fish at 82F. They did great.
KH 5.5/GH9. Food and general care, not packing too many fish or
leaving uneaten food in the tank, frequent large water changes etc.
As long as you are not too far off, you will be fine.

Too many folks think these are delicate fish to be coddled or they
will die etc.

Some folks want to breed them. If you want to breed, learn to feed.
Best to focus on that aspect and move away from plants some, you can
have floating etc, but generally lots of water changes/feedings, bare
bottom tanks etc if breeding is the primary goal.

Which plants do well at 84 F or higher ?

Most all plants do well, Lace plants peter out at higher temps. Some
plants don't do quite as well as they might at 75-78F etc but
generally most do pretty well.

I have these plants in the tank now :
Hygrophila species
North American water lilly ( species ? )
Amazon Swords
Dwarf Swords
Bananna plants
Hornwort
Anacharis
Which ones will be ok with the higher temp ?
Thanks in advance ;
Keith J.

If you are growing these now say at 78-80F, it'll not change
dramatically at 84F etc.
Might try Egeria najas in place of regular old Anacharis.

Regards,
Tom Barr