I've been running HB sidescan since it first came out, and have owned each generation as it became available. With each new generation you end up with a better product. I currently run a Helix 9 Mega+ G3N and an Onix 8 on the console, and my old 999 on the bow. All three are networked together along with the i-Pilot Link. I've got both the Helix & Onix transducers mounted on the transom, but mostly use the Onix for mapping. I don't fish a whole lot, rather I'm scanning looking for whatever's laying around on the bottom (60+ boats on the Madison chain plus tons of other stuff). I also Scuba dive so it all ties together. Personally I'll look closely at the Helix 9 G3 with the Mega+ imaging. The screen has higher resolution than your old unit and is a full digital system. Right now I think the G3 is around $1100, with the new G4 at the $1500 price that the G3 used to be.
Your old unit was analog and probably came with the subcompact transducer. This smaller transducer has smaller elements which yields lower resolution scans. The Helix 9 G3 Mega+ will give you much better detail. You get the three transducer frequencies, of which I use the 455khz or 1.2Mhz for searching (depending on depth, bottom composition and other factors). I don't recall if the subcompact comes with the 800khz or not. Regardless, personally I never use the 800khz, in my experience on the older units it didn't perform as well. You could upgrade to the full size (HD) transducer and this would give better scans (and give you 800khz, for what it was worth). On the older units the 800khz was a more compressed beam and the 455khz outperformed it. Your old 800 series did have a 7" screen, thankfully they got rid of their old numbering system and now the Helix 8 has an 8" screen.
The problem with losing bottom lock has to do with the size/shape of the transducer. On my old 999 I wouldn't lose bottom at top speed (40MPH). On the Onix which has a similar size/shape I lost it above 25MPH or so. Lowering it deeper into the water helped some. On the Mega+ it was useless, so I added the "high speed" 200khz transducer. Keep in mind that when you do this the 200khz is disconnected from the Mega+ transducer, so get the correct high speed transducer. Your mapping uses the 200khz beam as well as your 2D sonar.
Look at the specs between any unit you're considering & Helix 9 G3 Mega+. I haven't made the jump to the new G4 yet, can't justify it at this point. But the higher the resolution of the screen will make a sharper scanned image. On older models if you went to a bigger screen you didn't necessarily get a higher resolution display, in fact some were the same screen resolution. So a bigger screen on these units would be easier to see, but more "blocky".
If your old 800 series has networking you can connect an (expensive) cable between it and your new Helix "N" series. This would allow you to share waypoints between the two units, but not sonar. So if you're using the old 800 for mapping you can mark a waypoint on either unit and it will show up on both. You would need a separate mapping card for each unit also (can't share LakeMaster or Navionics maps). Also, the mapping card in your old unit is probably a full size SD card (depending on when you bought it, it may be a micro-SD card in an adapter). The Helix G3 units use micro-sd cards. Thankfully, since I remove my cards after every trip out (to manage my navigation data) these unit have nice slots that pop the card out far enough to remove them, and the slots are on the front of the locator. The smaller Helix's, up to the G2, have the slots on the side of the locator, and they basically suck in getting the card out. Not sure if the smaller G3's are this way or not.