if your current payroll application is running as single tenant application then you have to build the necessary support in your system to support multi tenancy such as how you want to isolate the date between the tenants in storage level and how authentication should be handled for different tenants and customization's you want to provide in your app e.g separate site logo for each tenant like that there are many things to consider during the design. There is no direct way to convert the single tenant app to multi tenant system.

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There won't be much different in deployment perspective. But you will get additional cloud benefits such as auto scaling of resources , no maintenance worries, basic fail overs and so on.. Not sure what kind of specific information you are looking for, could you be specific.

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