My company has all of our frames and files set up to use British National Grid as the projection. However, when adding in basemaps such as google imagery it becomes painfully slow and virtually unusable. After some tweaking I found that changing the data frames projection to "WGS_1984_Web_Mercator_Auxiliary_Sphere" this solves the slow loading issue, it all becomes super quick. My question is, what knock on effect will this have if I start creating shape files and maps to this new projection? Will shape files I make in this new projection still work on my old files that used British National Grid? Is there anything else I should know?