If I'm a bred and born ashkenazic male, who's been taught to follow the Mishna Brurahs Psakim all his life. Would it be significantly different to follow more or less by the Shulchan Aruch Harav? Leaving aside Minhagim issues.
Historically Lithuanian Jews followed Chaiya Adam until fairly recently. Polish Jews followed the SA HaRav. The MB gained halachic reign sometime after 1980. Ask anyone over 40 who is ffb what they learned halacha from as a teen. We are not bound to follow the MB even though he is one of the greatest commentaries to SA OH and aside for halacha kBasrei.
I added the "Hashkofoh" tag to the question because it is not Halachic per-se.
Following Halacha is not about the amount of Halochos but about one specific approach. So one who follows only Kulos (lenient) Psakim of several Poskim just as one who follows the Chumrot (stringencies) of different Poskim, are both fools.
It is more common with Baalei Teshuva that have no family tradition choosing their way in Judaism. I did it once and my Rabbi Z"l advised me to choose freely but eventually lock on one approach.
For those who have a family tradition, esp on the father's side, one has to follow it under "שמע בני מוסר אביך". In such a case, switching from M"B to ShuA Harav might raise the question of breaking a vow (as the tradition is considered sort of a vow).
To sum it up - one can either "convert" to Chabad (probably requires a simple התרת נדרים) and then follow the ShuA Harav in full or use the ShuA"H as a reference for resolving Halachic disputes (as most Ashkenazi do) but not follow it in cases that contradict the MB.
When the alter rebbe made shulchan aruch harav he made it for everyone (btw he made it becuase the maggid of mezritch told him to do it, and the vilnan gaon wanted to make a shulchan aruch he didnt, and he said its because he didnt get permission from shamayim) in the back of shulchan aruch harav there is "piskei hasiddur" which was written later by the alter rebbe for chabbad chassidim, his shulchan aruch was for all yiddden.
Even if you dont follow shulchan aruch harav, i suggest you learn it just because its such a monumental work, and you could be medayik in his lashon to see who he holds like, who he is negating etc.