Switch MySQL lag slave to new master and keep lagging (5.1)
DISCLAIMER : Not a user of mha4mysql.

SUGGESTION #1

Hang the Lag Slave off from the Standby Server. How?


Step 01: Stop all Writes to the Master
Step 02: Run SHOW MASTER STATUS; on the StandByMaster
Step 03: Run mysqlbinlog against the binlog from Step 02
Step 04: Acquire timestamp from given position
Step 05: Subtract 43200 sec (12 hrs) from the Timestamp acquired
Step 06: Locate position just before timestamp acquired
Step 07: Using StandByMaster Host IP, Log File and Position, run this on Lag Slave

STOP SLAVE;
CHANGE MASTER TO MASTER_HOST='...',MASTER_LOG_FILE='...',MASTER_LOG_POS=...;
START SLAVE;



All manual steps. I'm sorry.

SUGGESTION #2

Setup the Lag Slave under one of the Slaves PERMANENTLY. You'll never have worry about moving the Lag Slave between Masters again.

SUGGESTION #3

Setup the Lag Slave under a Blackhole Slave. You'll never have worry about moving the Lag Slave between Masters again.

I hope these suggestions helped or at least got you thinking.