Moved my Cobra Health Ins costs from sched A to sched C for 2017. What forms do I need to send in? (1040x, 1040, sched A and sched C)

Are you amending the 2018 return ?

No 2017 return

It's not on the Schedule C, it's on line 29 of the 1040

Send your 1040X and any forms that changed, which would be your 1040 and Schedule A..  There may be others, which depends on what else in your return. You'll have to look to see what other forms may have changed.  Send any forms showing your Health Insurance paid plus any forms that show tax withheld.