When MetaFont generates a font it includes a checksum in the font bitmap file, and in the font metrics file (TFM). (La)TeX includes the checksum from the TFM file in the DVI file.
When dvips (or other DVI drivers) process a DVI file, they compare checksums in the DVI file to those in the bitmap fonts being used for character images. If the checksums don’t match, it means the font metric file used by (La)TeX was not generated from the same MetaFont program that generated the font.
This commonly occurs when you’re processing someone else’s DVI file.
The fonts on your system may also be at fault: possibilities are that the new TFM was not installed, or installed in a path after an old TFM file, or that you have a personal cache of bitmaps from an old version of the font.
In any case, look at the output – the chances are that it’s perfectly OK, since metrics tend not to change, even when the bitmaps are improved. (Indeed, many font designers — Knuth included — maintain the metrics come what may.)
If the output does look bad, your only chance is to regenerate things from scratch. Options include: flushing your bitmap cache, rebuild the TFM file locally, and so on.