This, 100x this. You and your team don't just have to build an understanding of the code (e.g. frameworks, patterns, DB's, etc.) but the app itself so you can

This, 100x this. You and your team don't just have to build an understanding of the code (e.g. frameworks, patterns, DB's, etc.) but the app itself so you can truly understand it's purpose. Your rewrite won't (hopefully?) also be 100mm lines so being able to understand the high level purpose of the app completely and then diving deep from there, you may (hopefully) find many places where the system can be simplified. Are you really not exaggerating? 100mm lines of source? Yiiiikes...