If they are return air vents, lowering the grilles is as easy as cutting drywall. If there is ductwork connected to the grille, It is a more difficult relocation, but not terribly so. A sabre saw with a metal blade will cut the ductwork & drywall at the same time. Butt the grilles up against the bottom of the crown for minimal patching.

Dropping the air is the best bet, dropping the crown can work but you will need to place back-ups and it is a lot of extra work.

Another option is to build the crown out around the grill as well, go to www.installcrown.com and take the link to the forum I put a drawing of how to do that type of return on there. Use the forum search, or if you can't find it ask I'll point it out to you.

Also in the crown tutorial you'll find the steps to drop crown as well if you decide to go that route. Cutting the backer to the spring angle etc.

http://www.altereagle.com/

http://decks-ca.com

http://kingofcrown.com

Alter Eagle Construction & Design

Thanks guys...will check into lowering first then maybe building a box. Lowering might be difficult since there is no attic space to move from above. We'll see. Thanks again!