A quick search of the internet shows that the regex capabilities for Total Commander follow the basic regex syntax so the followion g may work. However I have no way to test these.
The simplest way would be to substitute all "-" characters with ".". This would not need the regex capabilities, but would also change names such as
which may or may not be a problem.
A regex pattern that would find dates of the structure you have shown would be
and yo could then use a replacement string of
However, you need to be careful that this will find every instance of this pattern of digits anywhere in the file name (and type presumably). If this is the case, then you will need to show us the full pattern of the way the files are named.
By the way, I'm not sure if Total Commander will rename the whole file name or will allow a partial name substitution - I've assumed that it can as this is the way that the vast majority of regex engines work. Let me know if this is necessary.