If you have already filed a joint return, then amending to a Married Filing Separate return may not protect your tax refund if it is seized to pay your spouse's debt. However, you can amend a Married Filing Joint return to a Married Filing Separate return, but it must be done by April 15th.
You should consider filing an Injured Spouse form instead. If you receive notice that your refund is being taken for a debt you are not responsible for, then the Injured Spouse form can be filed on its own to reclaim your portion of the refund.
Take a look at the following information to learn more:
Injured spouse relief
How do I file Form 8379, Injured Spouse Allocation?