When you move from one state to another, for that tax year you file as a part-year resident in each of the two states. See this TT help reference on that topic: How do I file a part-year state return? (intuit.com)
All your income as a PA resident is taxable by PA; all your income after becoming a CA resident is taxable by CA. You become a CA resident for tax purposes when you begin living in your new home in CA. Note that your income after the move may continue to be taxable by PA as well as by CA. Here is PA's rule regarding their taxation of telecommuting income:
https://revenue-pa.custhelp.com/app/answers/detail/a_id/3597/related/1
If you do have to pay PA after the move, you'll be able to take a credit on your CA return for tax paid to PA on the income that is taxed by both states. Since your post-move income will be 100% taxable by CA in any scenario, and CA's tax rate is higher than PA's, I suggest you ask your employer to withhold CA taxes. If they are unable to do so, you should make quarterly estimated tax payments to CA after the move.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
