Is it wrong to use one backend when developing another front end?

I have an MS Access application heavily coded with VBA and like 100 forms. I want to bring it to a C# ASP.NET MVC architecture. The current back end is a dedicated on premiss SQL Server Standard 2014 version. And there are 40 users.

The first step I wanted to take was to migrate the dedicated on premiss SQL Server to Azure. Then once the datasource is accessible online, I wanted to keep using the current MS Access app while coding the ASP.NET MVC app to slowly replace the Access app piece by piece.

This means I'd have users in the MS Access app, and possibly users in the ASP.NET app at the same time. I've been advised this is a bad idea by a developer who is helping, but wasn't give much details as to why.

Can anyone teach me or point me in the direction of why this approach has pitfalls? What I don't understand is I have a dedicated SQL Server already, and 40 different MS Access front ends all connecting to that. How is it much different than having it on Azure DB and letting an ASP.NET MVC front end also letting the MS Access front ends manipulate the same database until the ASP.NET app is fully completed?

Thanks for the help!