Prevent the personal union with Poland?
That would mean there would not be a union with Poland, I believe that Lithuania would likely be under the Nordic Sphere of Influence rather than being a part of Central Europe.
A union between Lithuania and Poland might or might not happen; a kingdom of Lithuania doesn't make that any more likely or unlikely. However even IOTL it depended on a few events and wasn't a given, so it might very well be that this hasn't any effect on the chance this would happen.
As of sphere of influence, this could be a larger role for the Nordic countries, but certainly in the beginning they mostly had contact with the Livonian order and thus indirect the HRE, furthermore there will still be contacts with the most nearby other Catholic kingdom, Poland, too. Under Mindaugas initial expansion was mostly eastward, but Lithuania also had issues with the Livonian Order, so potentially in the long run, they might be able to (directly or indirectly) include more, most or all TTL Baltic states (now IOTL these are Lithuania, Latvia and Estonia) in their kingdom. (Historically Baltic) Prussia also belonged to this Baltic region). Such a Lithuania might be more focused on the other states on the Baltic Sea. Though I might add that Central Europe can be a broad term.