Yes - Older deployment clients (UFs) are compatible with newer deployment servers.
Older UFs are also compatible with newer indexers/HFs:
https://docs.splunk.com/Documentation/Splunk/7.0.1/Forwarding/Compatibilitybetweenforwardersandindexers
(The inverse of both of these is not supported, although it works quite happily)
I have always started with indexers (or a master in a cluster) and dealt with search when the index is up to date.
The logic behind this is that older SHs will still work even if they are spitting warnings in the log, but the bulkof the work (excitement) is in the indexers. - If You have issues with your indexers you will almost certainly be rolling back everything - I like to find our early on in the process if thatch likely to be the outcome (it never has been). Next I do search, then DS/Lic and finally HF/UFs
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