Permissions when migrating to new install

I have an iMac with 1TB internal drive, running Yosemite, multiple user accounts.

I want to create an external SSD with El Capitan, and use that as my boot drive. Trying to figure out how to make this happen.

1. Buy 250GB SSD and UASP-supporting USB3.0 enclosure
2. Download El Capitan and install it to the SSD
3. Boot from SSD
4. Copy /Users/johnsmith and /Users/janesmith and /Users/shared from the internal HDD to a spare external HDD
5. Erase the internal HDD
6. Create Johnsmith and...

Permissions when migrating to new install