Don't you have a Cluster Master too?
The order should be:
1 - License
2 - Cluster Master
3 - Deployer - Upgrade it but keep it shutdown until you upgrade your Search Heads, so they don't contact a Deployer in a different version
4 - Search Heads
5 - Indexers
6 - Heavy Forwarders
If you have a multi-site indexers cluster and you're not willing to shutdown all sites to do the upgrade in all of them at once, for it to be a Splunk Supported upgrade you need to have intermediate upgrades at 7.0, 7.1, 7.2 and finally 7.3. If you can shut all indexers down, then it would be a straight upgrade to 7.3
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