Unfortunately you would have to "Re-Pass" either the ATPL or IR exams.
You would however only be required to do training up to the level as decreed by the Head of Training at a Ground School. A massive concession.
Its amazes me that the "authorities" expire these exams so quicky - they clearly have no faith in the degree to which the knowledge, understanding and application of the taught content is retained.
However the average pass mark has risen relentlessly since the question banks got into the www system - if you know what I mean, and so you should have little difficulty repassing - although handing more cash to the CAA is a tad irksome.
The whole thing will be an exercise in futlility but acts as more useful revenue for the CAA
There is no upgrade path from CPL->ATPL or IR->ATPL either