Should I be changing my engine?

I have a 1997 Dodge base Caravan with over 186,000 miles on it. [b:3811627689][u:3811627689]It has a 4 Cylinder, 2.4L w/ automatic (matched as it came).[/u:3811627689][/b:3811627689] I really need an engine with more horsepower BUT I cannot afford a new vehicle. THEREFORE, I am thinking of changing out my engine package for a larger engine,[b:3811627689][u:3811627689] preferably the 4.2L or 4.6L that was also used the same year.[/u:3811627689][/b:3811627689]
If I do so, what types of problems am I going to run into?
I do not plan on changing the AC unit unless it is necessary.
All advice will be fully appreciated.
Thanks, Horace[u:3811627689][/u:3811627689]