It can be done. Just have to find someone who can and is willing to do the job. Builders will pass because their work load is already stacked and taking a one off job may not be worth it.
I have an SVI 5.4" irons gun that i wanted to convert to open major.
A few builders including SV said it couldn't be done and to just sell it and have an open built.
Some builders refused or didn't respond back.
One company took it on.
It was a bushing barrel gun with a full rail dust cover.
Replaced the bushing/barrel with a comped bull barrel, drill and tapped for frame optics mount, mill slide for a racker.