If this is E-6 film it is no problem - BUT - you need to communicate clearly with a professional lab. I shot E-6 films professionally for many (25+) years and often rated film at a higher ISO, then had the lab "push process" the film to compensate. The film is kept in the developer longer than normal. We also used to "clip test" E-6 film. The lab would cut just the first few frames from the roll of film, then process it according to the ISO (back then it was ASA) speed at which I rated the film. I would then inspect the processed clip test frames, and we would process the balance of the film based on the test frame results (sometimes push processing further, sometimes less). Important note: when I did this all of the exposures on the roll were identical. I am certain there are good labs remaining on both coasts, probably in Chicago.