If I understand you properly, these pages are not proportionate, so a full page ad would reduce approximately 91% horizontally and 82% vertically, obviously not an ideal situation. However, you don't mention the grid which is more critical than actual paper size, if you adjust margins and gutters you might be able to make a proportionate reduction.
Tim

The question is how you bill out your advertising. Per column inch or is it modular?

If it's modular, I'd just figure out the new sizes by doing the whole "1/2 page is this on the old stuff, now it's this." by just figuring out what a half page is.

OR, if your paper wants to keep the same number of column inches is per ad, then I guess it means redesigning all the ad sizes and just phasing out the old stuff.

Example...
The newspaper I ran we had our first issue as a tabloid, but every subsequent issue was a broadsheet (50 inch web I believe). I kept everything as "3 col x 10 in" or so, that way, when the ads that transferred to the next issue, all I had to do was go by my columns by inches.

Maybe that helps, it might just sound confusing... It is 9 am for me, and I don't think the brain is functioning yet.

I don't think you can do any sort of formulaic resizing when the aspect ratio also changes. It sounds like you'll be creating a new grid layout for the new page size, right? As such, the ad sizes will change in relation to that.

Hey all thanks for the responses. Its a dreary monday morning and I am sick on top of it all!

We charge by the column (i think).

I havent messed with the grid yet I will check on it later this week when I have some more down time and I hopefully feeling better.

Hey all I am feeling better now. I am spending this day try to figure out the re-sizing issue.

Tim, you said mess with the grid and gutters and maybe I can figure out a porportionate reduction in all my ad sizes? How would chaning the grid really effect anything?

Hey I think I did it. I kept it 4 columns and made the gutter .125 brought in the left and right margins a few smidges and now the ad sizes proportionally scale down to the their correct size (i.e. a quarter page is now a "quarter page")

I think. Now I only got to figure out an easy way of figuring out the new add dimenions sizes somehow.

Bah I spoke to soon. Quarter pages fit fine by now 1/2 columns dont. WTF? Should I tell my superiors its not worth my time? I guess I could go back and re-design every ad but I dont think they really want me to do that.

This blows.

Luis,
Try reducing your original grid by 87% (split between the two percentage reductions of the page size). This will change your margins and gutters in both directions. See how this affects the look of the page the side margins will be larger and the footer and header will be smaller (proportionately).
Tim