What sequence when changing the master MySQL server, but not IP?
I'm interpreting your scenario as follows:


You start with one master database and two replicas; both replicas are replicating from the master.
During the promotion, you want to set the original master to replicate from the promoted replica.
The second replica you leave alone to continue replicating from the original master.


The command RESET SLAVE on a host that is currently a master would have no affect since this command works on replicas.

The Change master statement on the master needs to be run using the log coordinates of the promoted replica at the point in time of the promotion. The best way to ensure this, is to set read_only on the original master, validate replication has caught up, check the log coordinates (show master status on the host you are promoting), then promote.

In order for the second replica to continue replicating from the original master you need to have log-slave-updates enabled on the original master, otherwise it won't receive anything written by the new master. You can quickly test this by creating a new dummy db or table on the new master and validate if it replicates down to the extra replica.

If I were setting up for this

Prerequisites

All hosts should be running with log-bin, log-bin-index, log-slave-updates, relay-log, and relay-log-index. Replicas should be running with read_only set to true. Application users and standard database users should not have the SUPER privilege (e.g. ALL PRIVILEGES on *.*).

Promotion


Apply a maintenance page and stop background processing if applicable.
On the existing master mysql -e 'set global read_only=1'; select sleep(1); show master status;
On the existing master, set read_only = 1 in your mysql configuration file in case the host is restarted.
On the replica to be promoted mysql -e 'show slave status\G'| egrep 'Exec|Relay_Master_Log_File'
Compare the coordinates of the master to the replica coordinates to ensure they match. Until they do, your replica is not in sync, wait or investigate as needed.
On the replica being promoted mysql -e 'show master status; reset slave all; set global read_only=0'
Update the replica being promoted configuration file to read_only = 0.
On the original master run the start slave command with the coordinates from step 6, and the start slave command.
Redirect your application and background processing to write against the new master's IP address, and restart your services.


Hope this helps.