Replacing Paths In A Filename Collumn

[[Disclaimer: I am by no means a highly-skilled DBA. I have just enough db skills to get my job done, which involves light sql server and oracle tasks. I appreciate the help, guys.]]

I am trying to replace the path section of filenames listed in a collumn of my database. The filenames are UNC paths, and all files have 8 digit filenames with the same extension (they are all tif images). The length of the paths in the filenames will change, however.

Here is what my old paths look like:

\imageimagesNYY-MMABCDEFGH.TIF

..and here is the new path (all of the images are being consolidated into one directory):

\als-imagealscom31imagesdbABCDEFGH.TIF

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I have tried using the following query to do this:

update image set filename=replace(filename,'\imageimages50-08','\als-imagealscom31imagesdb')

This returns insanely crazy results like 460239 row affected, when it should be about a thousand.
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I have also tried the following:

update image set filename=right(filename,12) + '\als-imagealscom31imagesdb' where left(filename,21) = '\imageimages50-08'

This affects zero rows.

Please guide me down the one true path. My coworkers are clueless and I'm quickly running out of disk on the old box.

peace,

-jake