Under such conditions most likely water molecule will be substituted by chloride-anion according to the equation:

[Co(NH3)4(H2O)Cl](2+) + Cl(-) = [Co(NH3)4Cl2](+) + H2O

The color of solution will change - I don't remember the color of [Co(NH3)4(H2O)Cl](2+), but [Co(NH3)4Cl2](+) is green. Actually, this is a method of synthesis of [Co(NH3)4Cl2](+): you can start with [Co(NH3)4CO3](+), for example, and under the action of diluted HCl it will transform into diaqua-complex:

[Co(NH3)4CO3](+) + 2HCl + 2H2O = [Co(NH3)4(H2O)2](3+) + H2O + CO2 + 2Cl(-)

Then if you add concentrated HCl you will obtain [Co(NH3)4(H2O)Cl](2+); finally, when you heat it up to 80 degrees the [Co(NH3)4Cl2]+ will form.