There has been, and will be, no change of aircraft voice communications to digital within the forseeable future. The technology is not up to the safety demands of serving mobiles moving at hundreds of miles per hour reliably. DSP is a reasonable interpretation of what they are talking about.
Another possible meaning of "digital connectivity" is that they are now using digital networking to link their transmitter sites and control centers (such as VOIP). They could also be referring to satcom, as "a system-wide technology refresh" would imply all elements of ARINC/ASRI communications - satellite, VHF voice, HF voice, ACARS, and HFDL.