I got an amended 1099-R from my pension fund today and I have already filed my taxes. What is the threshhold amount allowed befor it is necessary to file an amendment?

There is not a floor on the amount of income from a pension that you are not required to report on your tax return. As a practical matter, if you report the wrong income and the IRS becomes aware of it, they would normally send you an invoice for the additional tax due plus interest and penalty when they become aware of the discrepancy, and if it is sufficient in amount for them to bother with. There is not a set dollar amount for what they might be concerned about though. 

what changed and how much? 

You must make every effort to include all income in your tax return.  

• However, your tax return must be "substantially correct".  
• The IRS defines this as: "For individuals, a substantial understatement of tax applies if you understate your tax liability by 10% of the tax required to be shown on your tax return or $5,000, whichever is greater."

Also See:

• Accuracy-Related Penalty
• Guide to IRS Tax Penalties: How to Avoid or Reduce Them
• Avoiding Underpayment Penalties

@Mike9241 was correct in asking what changed and how much? 

I received a second 1099R from the pension fund that was for $1200. more and tax withholding was $280.  

As previously stated, there is no threshold amount.

 

If entering the corrected form in place of the old one results in tax due, you will need to amend the return, however you need to wait until the original has been processed. There is a chance the IRS will correct the return in which case they will send a letter notifying you of the change and asking if you agree. 

If the change results in tax, make sure that additional tax is paid by April 15th. 

 

HERE is a link about making a payment without a return in case you need to pay before amending.