gemmy,
You should not need to format or otherwise clean up your card.
As far as transferring apps, you should use something like Titanium Backup on the Eris (to back it up to the SD card), and then (again) use Titanium Backup on the new phone to restore it. If you haven't already done that, probably the only thing on the SD card is some of the app's data files. (You are looking to back up the application ".apk" file at a minimum - I think Ti Backup can backup both that and application settings to the SD card - so long as it is not a "protected app", but don't quote me on that - I've never used it).
Note that sometimes an application will not show up in the Market when it shows up for other phones - that can be due to the fact that the App developer knows that there is a problem with the App on certain Android releases, and "marks" the App in a way that it will not show up for users that have that have a phone that uses that version of Android.
I don't know if that is the case here - I don't know what version of Android the Incredible has on it. But anyway - there is a small chance that even if you are able to back the app up, it still may not work.
But, give it a shot and see what happens. Please report back here so others can learn from your experience.
eu1