trader_4 wrote:
Let's say it has the typical vapor barrier similar houses would have.
What is going to happen that is so bad, from just adding more
insulation? If this house isn't experiencing any problems related to
moisture passing through the vapor barrier that is there, what's the
compelling need to rip out the existing insulation and do spray foam?

The house I'm living in, I'm sure the vapor barrier isn't perfect.
It isn't perfect in new construction either. But I'm not seeing
any problems from it. And if the OP doesn't have problems, I'm
not seeing the necessity of the rip out and spraying in foam.

If it doesn't have a vapor barrier, that could cause it to matt down. It would
at least be worth pulling up a bit of the old insullation and checking for a
good vapor barrier.