Hi Gorwin -
Something you might want to try is when you increase the font size, also decrease the page margins. If you imagine the effect of "zooming in" on an existing page layout, you'd expect the white space around the edges of the page to shrink as the area occupied by the text grows. With some fiddling, you might get a good match between the existing pagination and the "bigger font" pagination with narrower margins.
Other options include playing with the character spacing (Format menu -> Font -> Character Spacing tab) and the line spacing (Format menu -> Paragraph -> Spacing) You can set line spacing to "Exactly" and enter values other than the font size to finagle the amount of text that will appear on the page (i.e. specify a line spacing of 10.5 pts for a 12 pt typeface to squeeze lines together.)
You'll need to check for legibility with any changes you make, as adjusting these values can cause words to run together, lines to overlap, letters with descenders and ascenders to conflict, etc.
Another tip - Word can assign point sizes in half-point increments. When you are fiddling with font size and margin settings, it can be helpful to have the extra wiggle room of using using 11.5pt type instead of just having to settle for 11pt or 12pt.
There's a good chance some combination of these techniques will get you close to what you want. You'll probably still have to do some manual tweaking/page breaking when it's all done though. Good luck.