Do I need my lap band emptied before I get a revision? Is there a preference between sleeve and bypass after lap band?

Hello, I hope you are well. After a lap band it would be most appropiate a conversion to gastric sleeve. Although a decision between a sleeve and a bypass depends on various factors that you do not mention, a gastric sleeve is probably your best choice.

You really don’t need to empty the band if you have no complications.
The surgery can be done safely, as a one stage operation, where the band is removed at the same time as the revision.
Both options of Sleeve Gastrectomy and Gastric Bypass are good, but most surgeons including me recommend the Sleeve as it’s less invasive.

It doesn't look like you will need the Lap Band emptied unless you have some sort of obstruction. Normally, the lap band is removed during the first part of the revision surgery. 
You also asked which revision surgery is best for you (sleeve or bypass). It would depend on your current BMI. If you have a BMI higher than 50 then a bypass would typically be the best option. Under 50 BMI you could go either way and it depends a lot on you current medical condition and weight loss goals. Talk to a bariatric doctor in order to get a better idea of your options.