Not to sound too cocky or anything, but if Isaac were me, I'd just let him switch primaries like it was nothing but there were pros and cons to doing that.
Sure, a tachi had more reach and power, akin to the advantages of a katana to a wakizashi, but there was this adjustment period of sorts when changing into something new, also akin to switching gun platforms—AR to AK and vice versa—and it just might be the thing that broke the camel's back.
Additionally, Isaac's fighting style was geared towards just doing straight-up melee—only using his gun less than 1% of the time—and on paper, having something to supplement your fighting style was key but in the real world, it could spring a lot of problems.