vla-put-stylesheet on all layouts?

I want to check for and old stylesheet name and push a new one to each
layout in a drawing as needed. It works sometimes, errors out sometimes
with automation error. I thought the problem may have been with pushing the
same name as the current name to a layout; hence the IF statement. Am I
missing something here?

;;;change old ctb's to new ones
(vlax-for lay (vla-get-layouts (vla-get-activedocument
(vlax-get-acad-object)))
(setq plotsty (strcase (vla-get-stylesheet lay))
newplotsty (cond
((= plotsty "OLDNAME1.CTB") "newname1.ctb")
((= plotsty "OLDNAME2.CTB") "newname2.ctb")
(t plotsty)
)
)
(if (/= plotsty newplotsty)
(vla-put-stylesheet lay newplotsty)
)
)

Thanks,
Josh
A2K, LDD3, XP