The clearest examples of the sound change [ln] > [ll] appear in Old Noldorin, such as ᴹ✶khalnā́ > ON. khalla (Ety/KHAL²) or ᴹ✶skelnā > ON. skhella (Ety/SKEL). There are examples in Sindarin as well: ✶melnā > S. mell (PE17/41). A note in Outline of Phonology from the 1950s attributes this change to Telerin:

rn, rm, lm all remained intact [in Quenya] and were favored groups; but ln became ld (T. ll) (PE19/100).

Given that this change took place in Old Noldorin and at a later conceptual stage seemed to be shared by Sindarin and Telerin, it seems likely this phonetic development occurred in the Ancient Telerin period. An identical change occurred at a similarly ancient period in Welsh, between PIE and Proto Keltic (WGHC/§99iii).

Conceptual Development: Given ln combinations in Gnomish such as G. ulna-, it seems unlikely this phonetic change was part of the earliest versions of the language.