A GTA can shift from RCM to FCM and, vice versa, but such option is possible only at the start of a financial year i.e. in April and not in the middle of the year. Here is extract of GST Council meeting.
Extract of Minutes of 20th GST Council Meeting held on 5.8.17
“Allowed option of 12% GST with full ITC under forward charge. 5% GST with no ITC will also continue. (However, the GTA has to give an option at the beginning of financial year).”
2. Also see Notification No.20/17-CT(Rate) dated 22.8.17 Serial No.(iii) amending Notification No.11/17-CT(Rate) dated 28.6.97 substituted the following language against serial 9 :-
CGST 6 % +6% SGST
Provided that the goods transport agency opting to pay central tax @ 6% under this entry shall, thenceforth (meaning in future) , be liable to pay central tax @ 6% on all the services of GTA supplied by it.”; In my view, the word, ‘thenceforth’ stands for a financial year because option is available.
3. GTA engaged in providing GTA services which are under RCM are treated as exempted supplies in the hand of GTA. So no ITC is allowed to GTA who supplies under RCM. GST is to be paid by the recipient of service. Per contra, GTA working under FCM will pay tax @12 % ( CGST 6% + SGST 6%) and avails ITC. If GTA (FCM) again switches to RCM on the commencement of a new financial year, the supply of GTA service ceased to be taxable service consequent upon the change from taxable (FCM) to exempted service RCM. GTA (RCM) is to be treated as exempted service.
Regarding the reversal of ITC, proportionate ITC (remaining unutilized) has to be reversed/lapsed in terms of Rule 44 of CGST Rules.

No ,However once you opted from RCM to FCM  you cannt change the option during the year

No ,However once you opted from RCM to FCM  you cannt change the option during the year

No ,However once you opted from RCM to FCM  you cannt change the option during the year