Using an external startup disk to replace the internal one?

Hello everyone,

I have a Late 2009 iMac 27" running OS X Mavericks that I wish to upgrade.

More specifically, I want to replace the internal HDD with an SSD, for this I've already got all the necessary tools including the hard disk sensor cable, 2.5" to 3.5" disk adapter, the SSD itself, ...

My plan to do the upgrade is:

1. Connect the SSD via an USB adapter
2. Install macOS Sierra to the SSD as an external startup disk.
3. Boot the iMac from the SSD over USB.
4. Use Migration Assistant to copy over my data from the internal HDD to the externally booted SSD.
5. Disassemble the iMac & replace the internal HDD by the SSD as described by OWC.
6. Done?

I'm not directly starting with the operation as I'm not sure of these things:

1. Step 4: Will Migration Assistant accept the mounted internal disk filesystem as a source? This is not documented in the support page. This post suggests it does.
2. Step 6: Can I use a (previously) external startup disk (the SSD) to replace the internal one without any changes to some bootloader? Is this "plug-n-play"?

I could just make a bootable USB stick installer, replace the HDD, do a clean install on the SSD. But then I'd need either a redundant backup of the internal HDD or a 3.5" HDD USB adapter to move the data.

Do you think this procedure would work? Thanks in advance!