updating records when giving one value to the top 1st record

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MySQL

Hi All..
I have Table Like This
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sno  name  regno
1    Jhon  3201
2    mark  3203
3    bell  3205
4    bean  3206

When I update sno 1 record regno to 2101 remaining all records Should be Like this
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sno name  regno
1   Jhon  2101 
2   mark  2102
3   bell  2103 
4   bean  2104

i Tried In This Way According To OriginalGriff Suggestion

delimiter |
CREATE TRIGGER testref BEFORE UPDATE ON test1
  FOR EACH ROW BEGIN
   UPDATE test1 SET regno = regno + 1 WHERE regno > NEW.regno;
  END;
|
delimiter ;

If i Do UPDATE like

update test1 set regno='2101' where sno='1' 

Then I Get Result Like

sno name  regno
1   Jhon  2101 
2   mark  2102
3   bell  2102 
4   bean  2102

But I Need To Get Table Like

sno name  regno
1   Jhon  2101 
2   mark  2102
3   bell  2103 
4   bean  2104

Am Out Of ideas Can Any One Help Me Please
Thanks In Advance