OK, I think you're still trying to understand why you shouldn't upgrade the frame.
Here is a walmart Schwinn, which may or may not be yours:


This is 7-speed, so the rear dropout spacing may only be 126mm.
The seatposts on some of these cheap bikes are often odd sizes like 25.8mm.
The geometry of that bike is likely terrible.
Yes, it is in fact much heavier a frame than a $400 bike, and flexier, with weaker welds.
Is the headset 1-1/8", or is it 1"? (either way cheap bike always get absolute junk headsets)
Are the brakes on yours attached with recessed bolts, or are they exposed nuts?
It has a quick release for the seatpost collar. That needs changing.

In order to upgrade that frame to something similar as a $400-$500 bike you can get used, or even new, which still won't ride as well due to the heavier frame and piggish geometry, you will need: wheels, tires, cassette, R. derailleur, F. derailleur, crank, chain, bottom bracket, cable set, shifters, and brakeset. Then, the parts like your saddle, seatpost, handlebars, and stem are super heavy on the Schwinn and may even be steel. A $400 bikesdirect bike will have better parts for those too.
You simply aren't going to get all these parts for $400.

Last spring my neighbor new to cycling bought a brand new 2009 Jamis Ventura Sport for $350 total. Or for example, for under $500 you likely can find a Cannondale CAAD5 with Shimano 105 used. Just don't try to upgrade.