All of your styles will not be affected, if you have created new styles with your buttons separate. If you just overwrote the stock image files, you will have to recustomize.
If you alter the templates directly, they will be affected.
The 6.7.3 Update not only updates the copyright information and the Powered by, it also fixes some bugs in the Import/Export, and a few operational bugs. Full Release notes can be found at
https://www.ubbcentral.com/support/version.php?product=UBB.classic#6.7.3 As always, you can ask questions on here.