If you get 20 MBps, i suspect you are on VDSL and are fairly close to the DSLAM. To increase speed you would need to get a second VDSL bonded to the first one which would double your speed. The second VDSL line is $20 extra.
It would also double your upstream from probably around 6 to 12 Mbps up.
Usually, there are already typically 2 lines coming to your house so no additional work outside your house would be needed.
I don't think new customers would pay less than you would (after the 1 year)
Sonic has to play the game of intro rates to compare favorably with other ISP. I guess one could compare that cost with advertising except that it costs customers less instead of making someone else richer.
Existing customers should not look at price difference between new and existing customers, you should look at the value you get out of it, and is it worth it to you.
The benefit of the 2nd bonded line for DSL may not be obvious unless you have a large household and everyone is online at the same time downloading large amount of data. 20 Mbps is a lot already.
People on ADSL2+ pay the same rate and only get 3 or 6 Mbps, people on fiber also pay the same rate.
Do you know when is fiber going to be available where you live?
Also: are you using sonic's modem or your own? will it support the bonded lines? Will rental of the 2 line modem be the same price? i think it used to be $6.50 vs $10. I have always used my own modem so i cannot confirm. Using your own modem could be a way to reduce your bill if you know what you are doing.
Sonic should have called you when they said they would though.