Even though done manually, there must be some thought process behind why the cell is bolded.

Maybe the cell is less than or greater than a number...
Maybe the cell contains a certain word...

If you can bold by some type of search/pattern, then you can easily use that criteria in your if statement and further use conditional formatting to bold the cell.

Other than that, if there is no rhyme or reason when the cell is bolded, you will need to use VBA as Pecoflyer has pointed out.