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>If an OS is relased in 2016, will VFP core be modified.

I'd bet that the answer is NO. Vista is almost guaranteed to be the last OS VFP will have any support on.

If you still have a lot of work to do before release I would consider how long you intend to market the core of this application and your customer base. If the core of this app is expected to last for several years then rewrite in another technology should be highly considered. If your customer base (for an accounting application) has other technology options, you will be in a serious bind proceeding with a VFP solution when competitors will use the status of VFP as a BIG downside for your app.

Hopefully you are at using SQL server or another secure/stable/reliable database engine as the data store - this will be a big selling point for your type of application!

Just my opinion.