What fluid was used when the transmission oil was changed? And believe it or not, worn clutch parts can for sure cause that. The clutch is unable to fully disengage, making the transmission grind when it should be shifting smoothly. The worn clutch is much more likely since the problem came on rather quickly.
As far as what parts need replacing on the clutch, I would assume the clutch disk and possibly throwout bearing. I'm sure others will know better. Best of luck oh and welcome!