In your old system, you will find that all the speakers are detachable. When you buy a new system with 7.1 output system, you can always buy separate speakers and attach it. For that you will have to work out the connections or for perfection can hire a professional. I would suggest not to waste your money hiring anyone, as it is very simple.
When you have your 7.1 system, go through the manual for the best acoustic placement of the speakers. The speakers will be named as front right, front left, rear right, rear left, etc. You might note that we love to hear low frequencies(BASS) from front, and high frequencies from each side. Well you already have 5 speakers. Check out the center speaker. Probably it should be a woofer. It is equivalent to a sub-woofer. You can utilize it or get a new sub-woofer from the market, which is available as spare(You might not get a branded one). Well you can use the other four speakers for the rear and front speakers. To get the entire effect of the 7.1 system, you have to fix three more speakers, at the other three locations specified by the manual. This you can do when you can mobilize more funds. As these systems are very costly. It might also be noted, only DVDs with supported audio format(7.1 or more) will yield the best result. Others, like mp3s and normal stereo audio will give output which will be similar to the output of ur present system. So, think before you upgrade.
Audio frequency range : 20Hz-20kHz
Sub-Woofer : very low audio frequencies(usually 20-100 Hz.)
Woofer : low audio frequencies
Mid-Range : medium band of audio frequencies
Tweeter : very high audio frequencies
It might be noted that the dialogs are thrown in a ratio from the four corner speakers to produce a effect of the position of the speaker(male/female). The speaker(male/female) might even be at the back. Even for helicoptor sounds and other mid range sounds.....the above settings can produce a moving effect.
Try removing and attaching the speakers in a combination to understand the effect yourself.It will be fun.
I am an Electronics Engineer.