Maybe try the new effect in AE7 “Smart Blur” (30 day demo at Adobe).
Or Remove Grain in AE6.5.
Or I think the best solution and fastest is to Re-encode the WMV’s to a larger size with Windows Media Encoder 9 which will smooth itself.
Of course you can’t get any better quality than you had in the smaller wmv’s, but if the smaller ones weren’t pixilated, then the new 720×480 shouldn’t be either.
https://www.microsoft.com/windows/windowsmedia/9series/encoder/default.aspx
It’s free.
Here’ a step by step if you haven’t use the WMV Encoder before:
Convert A File
Source File; Your smaller wmv
Output File: choose a name
File Download – Computer Playback
Video, Audio: the second choice in the dropdown, DVD quality (2 Mbps VBR) should be fine (it’s 640×480, but we’ll change to 720×40 in the properties panel below)
Next – nothing to change
Next – uncheck the box at the bottom – Begin converting when I click Finish (so we can change the 640 to 720)
Finish
Now click the Properties Tab on the tool bar.
Video Size Tab – Change the Width from 640 to 720.
Hit the Apply button.
Click the start encoding button on the toolbar.
I re-encoded a 480×360 to 720×480 and the wmv9 codec smoothes the pixelation you get from going from wmv to AVI.
I have AE 6.5 and it can use the wmv9’s fine.