Changing page extentions (.htm to .php/aspx)

How to avoid dup content filter

[jacuzzi]

I changed over my pages from .htm to .aspx to use some dynamic content. As the .htm pages were indexed and ranked well I left them alone and just changed all the links to point to the .aspx pages (which were exact copies of the .htm pages but can serve so dynamic stuff as well)

I was hoping Mr.Google would drop the .htm and reinstate the identical .aspx pages. But that hasn't happened. I might have fallen foul of the dup content filter.

What should I do? Remove all the .htm pages and accept the collapse in google traffic until the new pages get ranked (yikes!)

Any help/advice much appreciated!