TFTP has some interesting advantages over regular FTP when it comes to IPv6.
At a glance, notice how the packet exchange is identical to IPv4.
In a download, the client sends a read request,
the server sends the data down to the client,
and the client acknowledges the data being transferred.
The UDP port numbers used operate identically to IPv4,
which is true for all FTP variants.
You can check the packet captures for yourself,
but the contents of the TFTP messaging contains no payload
information about IP addresses of any kind.
As such, TFTP is completely IP version agnostic.
I'm drilling into the data packet so you can see the
R1.cfg file being successfully transferred.
The upload operation isn't much different.
Just like IPv4, the client sends a write request and the server acknowledges,
clearing the client to begin sending traffic.
The client uploads the file using data packets and the
server acknowledges a successful transfer.
Regardless of direction,
the TFTP header contains no IPv4 or IPv6-specific information,
and thus is unaffected by changes in layer-3 protocol.
For completeness, I'm showing the data carried in the TFTP upload, which is the contents of R3.cfg.