EP3 3rd gear grind

so ive had my civic for about a year and a half or so now (bought it with about 77k, now it has 93k), and its starting to develop a 3rd gear grind. but it only does it when i go from 2nd to 3rd, not from 5th to third, or from 4th to 3rd, or when i baby it in. anyone have any ideas what it could be? just change the trans fluid? or would i have to get it rebuilt? and if i do have to have it rebuilt, would it just be easier/cheaper to get a new transmission? if it would be better to get a new transmission, how difficult would it be to throw a 6 speed in it?

any help would be awesome.