I'm just starting to learn Music Theory and trying to practice time signature conversion. I understand the basics but I don't know how to practice because I don't know if what I write is correct or now. Is there some kind of web app that can check for you or is there some other method of checking?
I guess one possible way, based on your comment of moving from 4/4 to 4/2 time, is this:
If the numerator stays the same, find the adjustment to the denominator and perform the reverse operation on the note values.
In other words, you go from 4/4 to 4/2, meaning you halve the denominator. So do the reverse of this to the note values, meaning you'll double them.
Similarly, let's say you want to go from 6/8 to 6/16. You double the denominator, so now you'll need to halve the note values.
I don't know if there is an automated way to "check" for this specific thing, but there is at least one way that you can be reasonably sure that you have converted your time signature correctly, and that is to compare the measure numbers. For instance, if you are switching from 4/4 to 4/2, and if there were 5 measures in a melody in 4/4 time, then there should also be 5 measures in 4/2 time. You could also check, periodically, that the note that you expected to fall on beat 1 in some measure is still the same note on beat 1 in that measure when you change to another time signature.
If you were to play "Twinkle, twinkle, little star, how I wonder what you are," and in 4/4 time, the "how" was on beat 1 of the third measure, then if you switch to 4/2 time, the "how" should still land on beat 1 of the third measure.
I hope this helps!
P.S., This becomes a little more complicated if you switch from a simple meter to a compound meter, but that principle should more or less stay the same.
P.P.S., This becomes even more complicated if you are switching from one number of beats per measure to a different number of beats per measure. Like 4/4 to 3/4. In that case, counting the measures won't work as well. Changing from something like 4/4 to 3/4 is less an exercise in converting time signatures than it is in rearranging a piece of music!
Sibelius can do this automatically with the 'Double' and 'Half' transformations. Not sure what other software supports this.
It's pretty easy to check this in any notation software. Just plug the original and converted music into different files, then double (or half) the tempo as needed. If they sound the same, you did it correctly.
If I understand you correctly, you start with a melody and from a specific point of time you want to change the metre. So if this is the case, and you don't want to worry about converting note values, I would suggest to change the time signature but keep its duration constant, i.e. go from 4/4 to 2/2 or 8/8. Or change from 3/4 to 6/8. (This way, you can use the same medoly with the same note values, only the metric changes).