Possible to remove accumulated haze on fluorescence interference filters?
Its probably not from heat.  Filters are basically a glass substrate with some ionic compound deposited in thin layers.  These won't oxidize at reasonable temperatures, and furthermore since you have it on the emission filter as well, its even less likely to be from the source.  

More likely its just accumulated organic material from the environment.  7 years is a long time, and microscopes are exposed to a lot of chemicals and other substances which can gradually build up on optical surfaces.  

Most vendors have a recommended way of cleaning their optics, but usually it involves lens paper or lint free swabs and methanol (or sometimes acetone for more durable stains).  You must be very careful though, as you do not want to scratch the filter.    

Semrock for instance has a detailed guide here:

http://www.semrock.com/cleaning-optical-filters.aspx