The guitar would have to be made of balsa wood for that to be a problem. Heavy strings can bow the neck slightly which is a good thing for
slide,but those are very heavy, Kirk and I play with a 16 on the first string and a 13 set for the rest for slide on our electrics which is considered very heavy. Mediums on an acoustic would be a set of 13s so unless your going much higher than that,your guitar should be able to take it even on a cheap guitar If by chance the new strings did make it unplayable then take it to a teck to be looked at because your guitar would be seriously flawed for that to happen.