Unicode doesn't have ‘umlauts’, and (with a few unfortunate exceptions) doesn't care about meanings and pronunciations. From the Unicode perspective, what you're talking about is the difference between Unicode Normalization Form C:
U+00FC LATIN SMALL LETTER U WITH DIAERESIS
and Unicode Normalization Form D:
U+0075 LATIN SMALL LETTER U
U+0308 COMBINING DIAERESIS
Unicode calls these two forms ‘canonically equivalent’.
