Generally - if you amend your federal tax return - you should review your state tax return and verify if you need to amend it.
You are not necessary need to amend.
Whatever TaxAct said - is a generic note.
Some states allow to deduct federal taxes paid on state income tax return - because with additional federal credit - federal tax liability would be less - that might resulted additional taxable income on the state level.
See FTB Pub. 1008, Federal Tax Adjustments and Your Notification Responsibilities to California, for more information. See “Order Forms and Publications” on page 67.
Because you do not deduct federal tax paid on your CA tax return -you do not need to amend your CA tax return.
You may confirm the answer by calling FTB - please find here Contact Information.
Let me know if you need any help.
P.S. Please attach a copy of your HUD statement to your federal amended tax return - form 1040-X.
You may expect delay in processing up to 4-5 months.
It doesn't matter which original form you used.
The reason for amending - "adding the first home buyer's credit"
It might be said in the way you put above - should not be matter.
Add the form 5405 and a copy of HUD statement from your closing papers - that is important!.
Do not forget to sign the form 1040X
Please be prepared to wait at least four months because of delays in the IRS processing.
Tax Preparer
Taxes, Immigration, Labor Relations