This conversion is done based on the .NET culture, Kentico does not modify the output. The quickest solution is indeed to change the field type to Text. The existing values will remain in the database.
The only other way to change this is to modify how the .NET evaluates and formats this value. However I would not recommend changing the decimal point character globally, as you would run into issues with wrong currency formatting. You can use this code directly in the transformation and change the format just for the single field:
<%# Eval<double>("<FieldName>").ToString(System.Globalization.CultureInfo.GetCultureInfo("en-CA")) %>