I just got a 1650d without an antenna, I have a gp-31 on my boat now which uses the gpa-017 antenna and I was wondering can I reuse this antenna for the 1650d. Will I lose any features using it since in the book it calls for a gpa-018

GPA017 will work fine for the GP1650 for basic GPS fixing but your DGPS fixing will not work because your don't have a DGPS type antenna (like the GPA018 or GPA019).
So does that mean the position just won't be as accurate with the 17 as a 18 or 19 antenna?
It depends on the unit in question, but your GP1650D would be less accurate compared using the correct antenna.
Also are different chart cards available still for this?
Can a chart card have another map loaded to it from online?
The 1650 was available in 2 versions, one read Navionics cards, and the other read C-Map (Jeppesen). You can still get up-to-date cards from Jeppesen, but I believe they are no longer available for Navionics.
These cards are fixed programs from the cartography company and can not be downloaded on line.