If its just CSS changes then you will be able to simply copy over the original templates template.css into the template CSS folder of the Joomla 2.5 installation. We included a major part of the CSS needed for Joomla 1.6+ in our 1.5 templates for this very reason.
Ciarán
Actually Ciarán I have more serious problems than that.  Every since I went to Neptune for Joomla 2.5 I have lost all my "Prev" and "Next" navigation.  I've spent two days now culling the Joomla forums to no avail.
While chatting with Josh a comment he made got me to thinking and so I did a test.  Momentarily I switched over to Joomla's Beez20 template, moving two of my modules (Main menu and Almanacs) to Beez's left-top position.  Here is the screen capture:
I then switched both menus back to Neptune and did a capture:
To make sure I had everything set correctly I then did a screen capture of the template manager page.  I wanted to make sure that Neptune was "assigned".
Unless I am far off I think the problem may lay with Neptune tho' I'm not casting aspersions.
Cordially,
Wayne