Changing your W-4 will not change the amount of the credit that is paid this year. The limit on the credit is not your tax refund, but your tax liability. Your tax liability is the total amount of tax you owe after considering income, deductions, and other credits. For example, if you had $5000 of withholding and get a $500 refund, your tax liability is $4500. Your liability is still $4500 even if you had $6000 of withholding and get a larger refund, or if you had less withholding and owed a payment. Your tax liability for 2016 is shown on line 63 of form 1040.
If your liability is less than the credit, you will get the credit up to your liability in 2017, with the rest carried over to 2018. So you will get it eventually.
Adjusting your W-4 only changes the timing of when you get your money. If you add allowances, you will get more money in each paycheck and a smaller refund at the end of the year. But it doesn't change the amount of credit you would be able to receive this year.
