I switched from BAEC to IB+ for exactly the same reason and regret it bitterly. Why? About half of my flights have been credited incorrectly or not at all. IB customer service is dismal, the agents can't do anything but "send a reminder to the responsible department". It took more than 6 months to get all the flights credited and finally attain OW Sapphire again. Add to that not a single word of apology and no compensation whatsoever. I have by now transferred most of my miles back to BAEC - but even this caused a few problems.

While BAEC will not change the start date of your membership year, you might be able to credit the TPs of the first few weeks of your next year to the prior one. This possibility is mentioned somewhere in the BA forum, most likely in the BAEC Guide thread. Unfortunately I only learned about this after switching to IB+.