Not true, you can ask to change unions, happens all the time, there is no bar when there is a raid.
AMFA, raided the IAM at UA and NWA and won for the M&R group, no bar on representation.
The PFAA raided the IBT at NWA for the FAs and won, no bar.
The AFA raided the PFAA at NW for the FAs and won, no bar.
AMFA raided the IBT at WN for the M&R and won, no bar.
You can change unions without a bar taking place. The only time there is a one year bar is when no union wins with 50%+1.
And the IBT has refused to raid the TWU at AA for the M&R, good luck on getting them to raid at US.