To answer this, you need to know the sole lengths of the boots. It's printed on the bottom or side of the soles. Some 28.5 boots actually have shoter sole lengths than some 27.5 boots. You may be able to look this up on line or ask here, hoping that someone reading this will have the exact same boots. For example, I have Lange L10s in 28.5 (BTW, the .5 is immaterial, as the shells are only made in full-size increments, so a 28 is the same shell as a 28.5) and my sole length is 323mm on boots from 2+ years ago, and 325 on last year's model (IIRC, which I think I do)
After that, you'll need to know what binding is on the ski, and how far in/out the heel piece is currently adjusted, to fit the 27.5 boot, to determine how much room you have to move it. Different bindings have different amounts of available travel. Plus, if it's at it's limit right now, then you won't be able to move it any further.
do you have access to this pair of skis, or is this on the internet? If you have access to them in a local shop, take your boots, and see if you can adjust the heel piece enough to fit with proper forward pressure. Too much forward pressure will cause pre-release issues.