Because iWork '08 applications are 32-bit, I suspect that you are getting the not optimized dialog when you run it, not a dialog that says Pages '08 is not compatible — unless it is earlier than v3.0.3. Pages v8.2 is fully capable of opening Pages '06, '08, '09, and later Pages documents.
Your Pages '08 application may be located in /Applications/iWork '08 folder. Dragging this folder to the Trash does not entirely remove the iWork '08 application dependencies scattered around your Mac filesystem from the iWork '08 DVD installation. Removing these applications means that you have no means of opening the documents as you originally authored them in Pages '08. You can open these files in Pages v8.2, but features exclusive to Pages '08 may not appear formatted correctly, or even potentially excluded. Worst case, the document would not open. Then you would have a problem…
So, you can for the most part open your Pages '08 documents in Pages v8.2. However, any attempt at subtle change to the document will be met by a descending sheet in Pages v8.2 requesting permission to convert the document format from Pages '08 format to the newer Pages v8.2 format. There will be no going back to the old Pages '08 document format once this occurs.
A safety net would be to create a backup of your Pages '08 documents, perhaps even with an accompanying PDF too. Don't let Pages v8.2 touch this backup.