Does altering an existing font breach the copyright of that font?
If I take an existing font and alter it using Inkscape for example, am I in breach of copyright?
Usually that's fine. Most font licenses let you alter them in the artwork you are creating.
Some let you alter the font file itself for internal use.
Some let you alter the font file itself and re-distribute it (usually open source ones).
But it all depends on the license. Read the license.