Recovery :: Copy Large Database To Another And Keep Them Synchronized?

We have a situation that requires converting all INT columns from source database to BIGINT in destination.

By ‘copying’ term, I consider any operation like insert-select, merge, asynchronous replication or any other that will get all data synchronized until some moment in time, i.e. 2015-12-31 10:00 PST when downtime is planned. Before that moment it isn’t important if data in source and destination DB are in sync.

Copying process starts early enough to allow appliance of any technique/technology – there’s enough time to find best solution.

Requirements:

1. Destination database is superset of source, having all tables and all columns plus few more columns in few tables. Additional columns have default NULL or they are computed. INT columns in source are BIGINT in destination.

2. If source and destination are not in sync at the moment downtime begins, it is allowed to have few minutes to wait for synchronization. For instance, if transaction logs from last hour or so need to be shipped, that would be ok. If large table need to be synchronized by comparing row-by-row, it’s not ok.

3. Source database performance must not be compromised by any copying if it’s done before scheduled downtime. There are couple of options I have in mind involving MERGE command but its appliance might be limited by current DB design. Additionally, based on business logic, I cannot be absolutely sure if some row, created while ago, wasn’t changed meanwhile.

For some large tables, there is no indicator at all when any row has changed – I cannot rely on IDs nor there is column like ‘[date last changed] DATETIME’

I have been reading about various kinds of replication, i.e. log shipping but I’m not clear if any of these is applicable. Key points, if there is replication involved, are copying process must start from empty destination database plus synchronization must not compromise source DB performance So, how to do this using any kind of replication?