Greetings
Not even sure if it is possible in Factorio.
Q? If it is possible, is train upgrade on your roadmap. That is I send a train to the train foundry and an upgraded train exits and the old one is put into boxes for the user to fetch and dispose of

Thx for the mod

Should be possible but how to know witch blueprint upgrade the old one ?!

I've just create a new mod variante without bp and there it's should be possible.

Rather than doing an "upgrade" on a train, just add a decommissioning stop, or shed. Any train which ends there is scrapped - parts go back into supplies. The foundry shed acts as normal. That way the two operations don't need to be synchronized in any way. The fleet can be increased or decreased as needed. The operational benefit to the base is that the original can keep running its route, and moving product, until the replacement it ready to launch. You could even include an active provider in the decommissioning shed and return any product in the train to the logistics network.

this is more possible and realist yeah

In a much more manual version, I did this in version 1.1. I had a fan of tracks (think IRL switch yard) where I would place prints and have the bots build. The prints included a couple pieces of wood as fuel, since there never seems to be a shortage of wood. 1.1 didn't place trains in automatic, so after they were built I had to switch them to automatic. All prints included a schedule which took them out to a fuel point designed to provide the proper fuel, and to the proper wagon locations, avoiding cargo wagons, which will gladly accept fuel as well.
The decon was similar. I had another fan of tracks. There was a stacker before a single station which emptied any cargo and sent it back to the logistics network. Then the train advanced "cargo empty condition" to the fan and just sat. Every so often, when I remembered, or was making a new train, I'd use a filtered deconstruction planner to 'wipe out' the waiting trains.
Since it was all bot-based I could do it from the map view. Even before WUBE gave us "Remote view". To start the decommissioning all I had to do was at the "Boneyard" stop to the train's schedule and send it there.

new version is release with a recycle stop
was very hard for design :/