Quite a switch, and for reasons beyond me.
The grades required for business management degree would depend on the individual uni. Generally, top end unis would be asking for D*D*D*. Having said that, you can still apply for the business degrees with your current A Levels because business degrees generally don't require specific subjects so long you have high enough grades. You can also generally apply for a business degree with BTEC in any subject, assuming the uni would accept BTECs for their degrees.
If you want to apply for top end unis which are targeted for investment banking, you're generally better off getting high grades in your A Levels as oppose to BTEC, which they tend not to like as much. Which degree you do generally doesn't matter as much (it's not a career like medicine, dentistry, education, architecture, engineering where you need/helpful to have specific degrees).
Whether you can switch to a BTEC will depend on the individual college, not the uni. Whether the uni would accept your BTEC would depend on the specific course. Without knowing the specific business degree you intend to apply for, it's very difficult to say whether a BTEC in business would be accepted.
If you want more help on this, I would need to know the specific degrees at the specific unis that you intend to apply for.