OS. [rl] became [ll]; [rl] > [ll]
Based on a single example, it seems that [rl] > [ll] in Old Sindarin compounds, since OS. glawar-lin > S. Glewellin “Song of Gold” (PE17/61). This was not true of later or reformed compounds, since rl could appear in later Sindarin: aerlinn, Dagorlad, Forlond, etc. Compare this to Quenya, where [rl] > [ll] was the regular sound change.
Conceptual Development: There are no signs of this phonetic development in Noldorin or Gnomish, but it could simply be lack of examples.