How does it know which lines are chords?

Every word on the line has to be a chord symbol. That is deliberately strict, because a lyric line containing the word A or the word Am would otherwise be transposed into something that is not English. The cost is that a chart with chords written inline among the words is left alone.

Will my alignment survive?

Yes. Chords are replaced in place rather than the line being rebuilt, so the spacing that puts each chord over its syllable is preserved. Where a chord gets longer, from F to F sharp, the character count changes by one and the alignment shifts by that much, which is unavoidable in a plain text chart.

The key decides. Sharp keys take sharps and flat keys take flats, and getting it wrong makes a chart harder to read even though it sounds identical. F sharp and G flat are the same fret. There is no way to infer the target key reliably from a chord list, so it is a choice rather than a guess.

Chord Transposer

Move a chord sheet to another key.

Runs in your browserNothing you enter is uploaded, and it keeps working with the network off.

Chord sheet

Paste it as it is. A line counts as chords when every word on it is a chord.

Semitones to move

Negative moves down. A capo on fret 2 is minus 2 here.

Spell accidentals with

sharpsflats

Move a chord sheet to another key.

How to use it

Paste the sheet. Chord lines and lyric lines together.
Say how far to move it. Negative goes down. A capo is negative.
Pick sharps or flats. The key decides which reads better.

When you would use this

Transposing a chord sheet is a substitution with two things to get right, and most tools get one of them. The first is knowing what to substitute. A chord sheet is chord lines and lyric lines interleaved, and a lyric line containing the word A, or Am, or Be, looks exactly like a chord to anything doing a naive find and replace. The rule here is strict: a line counts as chords only when every single word on it is a chord symbol. That protects the words at the cost of leaving inline charts, where the chords sit among the lyrics in brackets, untouched. Mangling somebody's lyrics is a worse failure than declining an unusual format. The second is keeping the alignment. A chord sheet works because each chord sits above the syllable it changes on, and that positioning is made of spaces. Chords are replaced in place rather than the line being rebuilt from parts, so the spacing survives. Where a chord grows a character, F to F sharp, the alignment shifts by one and nothing can prevent that in plain text. Only the root and any slash bass move. Extensions ride along unchanged, because a m7b5 is the same shape in every key. Spelling is a choice rather than a guess. F sharp and G flat are the same fret and different notes on a page, and which is correct depends on the target key, which cannot be reliably inferred from a list of chords. A capo is a negative move: capo on the second fret is minus two.

Questions

How does it know which lines are chords?: Every word on the line has to be a chord symbol. That is deliberately strict, because a lyric line containing the word A or the word Am would otherwise be transposed into something that is not English. The cost is that a chart with chords written inline among the words is left alone.
Will my alignment survive?: Yes. Chords are replaced in place rather than the line being rebuilt, so the spacing that puts each chord over its syllable is preserved. Where a chord gets longer, from F to F sharp, the character count changes by one and the alignment shifts by that much, which is unavoidable in a plain text chart.
Sharps or flats?: The key decides. Sharp keys take sharps and flat keys take flats, and getting it wrong makes a chart harder to read even though it sounds identical. F sharp and G flat are the same fret. There is no way to infer the target key reliably from a chord list, so it is a choice rather than a guess.