From the TSP website:
Active Military? Your Roth Contributions May Stop Unless You Act— (December 3, 2014) If you are an active duty member of the Army, Air Force, or Navy* making dollar-amount Roth contributions to your TSP account, these deductions will stop on January 31, 2015, unless you act.
How your election requirements will change:
Starting January 1, 2015, a change in myPay will require you to designate your Roth contributions as a percentage of your pay, not a dollar amount. If you don’t comply with this change, then the Defense Finance and Accounting Service (DFAS) will not be able to process your Roth contributions. This change affects your Roth contributions only; your traditional contributions are already designated as a percentage of pay.
When the change will take place:
The new requirement will take effect January 1, 2015. You will have 30 days to change your Roth election from a dollar amount to a percentage of your pay. If your new Roth election is not received by January 31, 2015, then DFAS will not be able to process your Roth contributions until you update them.
How to make the change:
Log into myPay starting January 1, 2015. You'll see a special TSP section called “Traditional TSP and Roth TSP”—click there. Then, in the “Contribution from Roth TSP” section, you can enter the percentage of your pay that you’d like to contribute (10%, for example). Finally, click “Save” at the bottom of the screen.
*Includes Navy Reserve component who serve more than 30 days on active duty
Looks like you make your change in January to avoid the break in contributions!