Hi,
If the requirement is just to add one more stack member, then no need to change the priority
Stack master and switch priority conflicts are resolved in this order:
1. The switch that is already master remains unchanged.
2. The switch (or stack) whose master has a higher user-configured priority.
3. The switch (or stack) that is not using the default configuration.
4. The switch (or stack) with the more recent license or image version.
5. The switch (or stack) with the higher hardware priority or hardware version.
6. The stack with the most members (if two stacks are being connected).
7. The switch with the lowest MAC addresses.
http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/troubleshooting/switch_stacks.html#wp39384
renumbering will not cause any disturbance and you can do this as an additional task (for better housekeeping etc)
regards
PK