Optical Cable 7.1 and Bit Rate?

[preatis]

I have an older 7.1 receiver (Onkyo TX/NR1000) that has only optical inputs. I have a 7.1 room setup and a Samsung Blue Ray player.

My question is there any difference between movies that have the 7.1 PCM option or the Dolby Digital 5.1 EX on my system? I am of course re-encoding the sound to DTS (or should I) from the player because of the optical cable.

2nd Question is

I paid about $3,000 five years ago for my receiver and know that HDMI would be needed for optimal sound however I am reluctant to repurchase a receiver because of my investment. My receiver is 7.1 analog (multi channel ) input capable though. Would it be better to purchase a DAC from HDMI to separate analog outputs (or do they even make them)? I remember back in the day my old DVD player had the separate channel outputs options for the higher end receivers (the DAC was from the Player). Pending the answer form my first question, If it would make a sound difference (Discrete PCM) do they make a Blue Ray player with the individual Multi channel outputs?