Almost certainly they will drop. The current 3DS/2DS/XL's will not be able to play games that are only compatible with the New 3DS and XL handhelds.
This means:
A) Informed customers won't want the older consoles, unless they are cheaper in price, lowering the price.
B) Nintendo will want to get rid of backstock of existing models, and will lower the price and have retailers unload them with cheaper deals.
C) Existing customers who are upgrading, will sell their older 3DSs, lowering the price.
D) The only people interested in the older models at their current price will be collectors. They'll want them new and unopened, and likely already have the more common models.
I seriously doubt they'll hold their value.