The need to upgrade is purely based on how you use or plan to use a depth finder. If all you want is to know how deep the water is at the boat, then any unit that shows depth will work for you.
If you plan to find suspended fish or bait fish and know what you are seeing or be able to tell if there are fish in brush piles, then you need to upgrade.
The first consideration for me is pixels. I want at least 480X480. If you upgrade and color is in your budget then pixels and color. Color makes target seperation easier.
If you do a lot of casting toward the bank, a trolling motor upgrade probably makes more sense as Trackers usually are way under powered in that category.