Resusing my own poster abstract for another conference I have to write an (poster-) abstract for a conference in April. Back in November I already wrote an abstract for a similar conference. Since I have not gained many new results on the same research topic, I made some minor changes to the 'old' abstract and was thinking about submitting this 'reused' abstract for the conference in April.

Is that professional? Or is this even plagiarism, since I copied work that is already published (I am first-author of the 'old' and 'reused' abstract)?

I am not lazy, but the abstract that I wrote last year seems to be very good and I don't see the point in rephrasing it.
Assuming you're talking about a one or two paragraph abstract, the question here is not really about the abstract per se but the contents of the poster.  In my opinion, you don't need to worry about self-plagiarism per se from a brief poster abstract because it's not a real publication in the first place (if it were a 1+ page extended abstract, that would be a different matter).  

Instead, the important question is whether your research community will find it acceptable to have you presenting essentially the same material in two different conferences.  In many communities, this is OK, taking the view that it is essentially raising visibility of a result with different groups of people; in others communities, however, it may not be viewed as acceptable.  Consult with your supervisor to see what the standard is for your community.