The auditorium, which can accommodate up to 1,000 people, is having a new ceiling, enhanced electrical services and new acoustic panels fitted.
Work commenced in July and will finish in time for the start of the BIC's busy autumn season.
Phase two of the development, which includes finishing touches such as new carpet and doors, is scheduled for February 2010.
The project is being managed by south coast building company Greendale Construction.
"We are celebrating our 25th birthday this year and as with everything else in life, times change and things move on," said BIC head of conferences, exhibitions and events Clive Tyers.
"This is a significant investment and we expect to see a financial return easily within the next couple of years, as well as improving the service we give to our clients."
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