The following evidence is consistent with Main Street becoming High Street, but certainly doesn't prove it.
Earliest newspaper items found with search term:
"Main Street Alness": 1854 (the only hits are in this year)
"Main Street Bridgend": no hits for Alness (tricky search, lots for Perth)
"High Street Alness": 1882
"High Street Bridgend": 1887 [with evidence that it is in Alness]
"High Street Bridgend Alness": 1904
Also the 1854 items refer to an un-named public house to let. It specifies "the south side of Main Street, Alness (East End)".Looking at maps it sems that there were only two named public houses on the east side, the Commercial Hotel near the Bridge and the Railway Hotel to the east, both on High Street.