Hey Samuel,
Just to clear things up, the pre-approval letter is something a mortgage broker produces for you, to show a seller that you have been properly vetted by the broker (collecting income documents, understanding your debts, and credit profile). A prequalification letter means that your file has been properly vetted by a lender, and as long as you can provide them with the documents they have requested from you, then the financing should be secured.
If you get prequalified with a lender, but the circumstances of your file have changed, then you run the risk of getting your loan denied due to the change in circumstance. However, if your new w2 job is in the same line of work as your current job, then the lender will usually be okay with that and go off of the new income source. If you're making less money than before, they may reduce your maximum loan amount allowed.
On the flipside, let's say you go from w2 income to 1099 (self-employed) income, then you will run into issues due to insufficient work history.
Hope this helps!