Changing vst sound/instrument automatically as track plays

In LE5 I have recorded an instrument track. I do not have an external keyboard and I used the virtual keyboard to record a single track using Halion One. What I want to acheive is that when I play the track back then at set points, in realtime, it will change sound or instrument automatically, say from high string to low string, or from Halion One to a different VST. From the instruction manual I thought I could do this with inserting a program change, using automation, and the chase events function. My laptop cannot handle many individual tracks being open at the same time and I think to have it on one track which changes as it plays back is better for me. Is this possible? I have tried and tried. I am fairly new to Cubase. Many Thanks.