Ok, so I've tried searching and reading faq to find the solution to this thing, but failed, unfortunately. What I need is this, basically:
Etude Op.10, No.11 In E-Flat Major - Allegretto
switched to
Etude No.11 In E-Flat Major - Allegretto, Op.10
trouble is, there are 24 etudes (e.g. tagged No.1, No.2, No.3, etc.) and I've no idea how to include fields like No.number. I hope it makes sense 
You may create an action using a regular expression to divide the title string into parts and put the parts together into new sequence:
Actiontype 5: Format tag field
Field: TITLE
Formatstring: $regexp(%title%,'^(.+?)÷(Op.\d+),÷(No.\d+)÷(.+?)$','$1÷$3÷$4,÷$2')
... or ...
Actiontype 4: Replace with regular expression
Field: TITLE
Regular expression: ^(.+?)÷(Op.\d+),÷(No.\d+)÷(.+?)$
Replace matches with: $1÷$3÷$4,÷$2
Note: Replace one special ÷ character with one space character.
Input string:
'Etude Op.10, No.11 In E-Flat Major - Allegretto'
Result string:
'Etude No.11 In E-Flat Major - Allegretto, Op.10'
DD.20090902.2252.CEST
Thank you very much! (I also needed that).
I'll give that a try today.
I'll also try to learn how to do this kind of stuff.
Looks very powerful!
deus62