I'm not sure I understand the question: you can change the priority where you need it. If you are not sure, you can simply put it at the beginning of the procedure, if perhaps it is a very long procedure, and there is only one specific request that is prone to deadlocks.
Although a better solution is likely to avoid a dead end , if possible.
You may also notice that any change in priority within the reset stored procedure corresponds to the priority of the calling session when the procedure ends:
set deadlock_priority high go select deadlock_priority from sys.dm_exec_sessions where session_id = @@spid go create proc dbo.p as begin select deadlock_priority as 'PriorityBefore' from sys.dm_exec_sessions where session_id = @@spid set deadlock_priority low select deadlock_priority as 'PriorityAfter' from sys.dm_exec_sessions where session_id = @@spid end go exec dbo.p select deadlock_priority from sys.dm_exec_sessions where session_id = @@spid drop proc dbo.p go set deadlock_priority normal go