I assume you mean from a grow bag or bag of compost. You could but the compost wont have any nutrients left and it may contain diseases.It will be best to start again.

Nicky is right of course, unless you are talking about something in a pot that is perennial and established - in that case you only need to take out the top of the compost - top inch or so, as much as is loose,- and then replace that with new compost.

No

I put the contents of my containers and grow bags on the garden and use new for planting

As Drc says use it on the garden only.

I use the compost from my buckets which grew tomatoes and peppers last year,and add some fish and bone meal. Having fed the toms so well,I like to think that plenty of goodness remains in the buckets.I use a cement mixer.
I would not start seeds off in it ,but it is fine ( I think ) for potting on the seedlings into the 3" pots. After that,it ends up in the garden anyway,and slowly slowly is improving my soil.

Put it on the garden - far safer!