Failover and Failback SQL Server Log Shipping
If i understand correctly, you want to reinitialize log shipping from B to A?

If yes, then once you bring the DR site database (B) online you automatically break the LSN chain hence the restore log files will be skipped. The only way to reestablish log shipping is to remove log shipping setup completely, create backup of database B, restore at database A in stand by/readonly and then setup log shipping from B.