I have two legacy applications with huge source code base which should get integrated into one application. As my knowledge of the domain of the applications is quite limited and there is no testing documentation (only exploratory testing was done till now) , I want to prepare a set of test cases to cover existing application functionality. Afterwards I want to use these test sets after the merge of applications to assure that from functional point of view new app works in the same way as old apps and the changes in code didn't cause any regression.
I researched on this topic and found some methodologies which might be usedful:
ACC (attributes - components - capabilities) - this is rather manual process and depends strongly on the experience of a test engineer (https://code.google.com/p/test-analytics/wiki/AccExplained)
MBT (Model-Based Testing) - test cases are automatically generated using specialized tool, test cases quality depends on the quality of a model i.e. the things we didn't model won't get tested
BDT (Behavior Driven Testing) - test cases will be derived from user story acceptance criteria, again it's a manual process
What would be the best methodology to use in my case ?