I assume you've done concurrent filing.
Is it safe to switch employer after 6 months with a pending I-140 ?
The answer is no ****at this time.****
Are there people who have tried this with texas center and still got their green thing ? Yes
Are there people who got rejected at I-485 stage because they switched employer without an approved I-140 after 6 months of applying ?
I have not heard so far.
You can join part time after you get your EAD. But if you use it, you'll lose your current visa status. If it is H1-B then you won't be able to bring any dependents to USA.
Now our INS is going to issue another memo some time in the coming 2-3 months. In this memo they are expected to talk about when one can switch employers, what happens if employer revokes I-140, what happens if INS rejects I-140 etc.. etc...
So don't take risk yet