Pilot reported the inadvertent crossing of a hold short line due to unfamiliarity with taxiways after ATC had instructed a last minute runway change while on final approach.

Narrative

Landing at APA on IFR flight plan; I was cleared by Denver Approach Control for visual to 35R. I read back my clearance to approach; briefed the 35R visual approach; reviewed the Airport Diagram to familiarize myself with taxiways for 35R and set up an RNAV 35R into my Garmin 430W for lateral navigation assistance. After established on final for 35R I was instructed to contact Tower on frequency of 118.9. I contacted APA Tower with call 'Aircraft X final 35R'. Tower asked me to confirm I was cleared for 35R which I acknowledged. On stabilized final for 35R; at approximately one mile final I was instructed by Tower to 'change your landing clearance to 35L and change Tower frequency xxx' (from the previously assigned 118.9; I don't recall the new frequency). I adjusted my visual lateral navigation and moved slightly left to establish final on 35L; changed to the new assigned Tower frequency and called Tower noting I was on short final 35L. I was cleared to land 35L. My visual glide slope; which had been established for 35R; was slightly low for the displaced threshold of 35L and I received an altitude alert from Tower. I was well above ground and acknowledged the alert; adjusting my visual glide slope for 35L. There was no time to brief taxiways for 35L. After uncomplicated landing I informed Tower my FBO destination. I was instructed by Tower to exit 35L on B12 and proceed north on Taxiway B to B8 and hold short of 35R on B8 and remain on Tower frequency. As I was preparing to turn onto B8 I noticed a high wing aircraft departing 35L off my left wing in what seemed an extremely steep angle of attack; with Tower simultaneously calling an alert to the departing aircraft asking something to the effect 'Nxxx are you in trouble?' Distracted by both an unfamiliar taxiway which I had NOT been able to brief prior to landing and by the adjacent unusual attitude departing aircraft; I initially missed the B8 hold short line (which is immediately on turnoff from from Taxiway B and relatively far from 35R). The nose of the aircraft only crossed the hold short line before I braked. Most of my aircraft was behind the hold short line. Tower called me a short time later and instructed me to 'cross 35R without delay to A8 and contact Ground'. I did as instructed; contacted Ground after clearing 35R and was instructed A8 to ramp. I was clearly in error allowing the nose of my aircraft to cross the hold short line; but I also feel that a truly last minute change by Tower when I was established on short final to an unfamiliar parallel runway with subsequent taxi instructions to a very unusual hold short line at B8 contributed to my error. Also contributing to my error was distraction of an unusual and worrisome attitude takeoff just off my left wing. I the future I will brief all alternative taxiways for unfamiliar airport rather than my anticipated runway/taxiway. I feel ATC can help by not making short final runway assignment changes.