Changing a Close Period back to Future Entry

Whilst doing November's accounts someone accidently opened the January period in G/L (changing it from Future - Entry). They then closed it but now, having opened December I want to change it back to Future - Entry so that we can select it as a reversing period when posting December journals but it won't let me change it to anything other than Open (which I don't want to do yet). Is there any way of changing the period back from Closed to Future - Entry?

Thanks

Martin