Reimplementation from NAV 2009 R2 classic to Dynamics 365 Business Central Question?

Hi Folks -

One of our customer who are on NAV 2009R2 classic client intend to do a re-implementation in Dynamics 365 Business central. Their NAV 2009 R2 database is heavily customized with even NAV base code being modified or commented out to meet their business requirement. In 365 BC on premise I am aware that you can still modify base Nav code but MS will take away that in coming months and future upgrades will be a huge issue. Having said that my question is :

1. With the base Nav code heavily modified in customer database, In Business central can they get around standard NAV base code to get the same functionality what they have currently in Nav 2009 or do they have to change their business process? 

2. Can Extensions be used with such heavily customized database? If not what are the other alternatives that are possible?

3. Will data migration be a issue from NAV 2009 to 365 BC?

Any help is highly appreciated.