Just a guess, but I would tend to think that the part number appears
frequently throughout the document. Even though it is the only thing you
changed it stands to reason that you changed every instance.

If the horizontal specs of the characters in the first PN differ from the
parameters of the characters in the new PN, proportional spacing may very
well explain the reflow. IOW, WK49845 takes up more space per occurrence
than IU11011 even though it's the same number of characters. And if the
number of characters in each part number differs as well it would compound
the issue even more.

May sound far-fetched, but if the metrics make a big difference. If you want
to confirm the effect, just select a few paragraphs of text & watch what
happens as you change from one font to another - without actually changing
font size _or_ conten t:-)

I can't say for sure, but if *everything else* is identical - system, print
driver, etc. - and you did only the Save As & Find/Replace it's the only
thing I can think of. Hopefull some other ideas will come along.