There is of course a difference between abrogation and specification. By the latter is meant that one revelation explains in more detail or according to specific circumstances how another revelation should be understood.

Example:

Sura 2:183 says 'O you who believe, fasting is prescribed to you ...'

Narrated 'Ata' that he heard Ibn 'Abbas reciting the Divine verse 'for those who can do it is a ransom, the feeding of one that is indigent' (2:184).

Ibn 'Abbas said 'This verse is not abrogated but it is meant for old men and old women who have no strength to fast, so they should feed one poor person for each day of fasting (instead of fasting). [Bukhari, VI, No. 32.]

It is quite clear that the second verse (2:184) does not abrogate the rule of fasting from the first verse (2:183) but explains that in a specific case, that of feeble old people, there is a way of making up for the loss of fast.