Is it possible to change the bearings in a 1997 rim that uses a 19mm axle to bearings that will suit 25mm axle ?

Yes

52mm Outside diameter x 1" inside Diameter.

Fleabay is your friend

just remember the 3/4 bearings are wider than the 1" inch so you may have to change wheel spacers to suit

+ 1 I'm going through that issue at the moment, 3/4 bearings are much wider than the 25mm and its causing me all sorts of problems