1) determine the use cases of the different applications involved (ie, what is each one used for, how does it fit into the company's workflow)
2) treat each app as a black box, understand the major data flows involved (which data sources is it interacting with? is it doing reads or writes? which tables)
3) treat each app as a black box, and try to understand the interactions between each app and any external components (other apps, web services, etc)
4) identify the overall architecture, determine the class hierarchy for each app, identify the major classes and functionality
By this stage you should be ready for a rewrite. At no point do you need to go into the code in any great depth.
2) treat each app as a black box, understand the major data flows involved (which data sources is it interacting with? is it doing reads or writes? which tables)
3) treat each app as a black box, and try to understand the interactions between each app and any external components (other apps, web services, etc)
4) identify the overall architecture, determine the class hierarchy for each app, identify the major classes and functionality
By this stage you should be ready for a rewrite. At no point do you need to go into the code in any great depth.