I did some math to see how good Sorcerous Burst is on average, now that it's a d8. Because these are averages, I'm only looking at additional bonus dice added from the base dice, because the chances of rolling a second round of bonus dice is so minuscule (even at lvl17) that they don't affect the average damage significantly.

From levels 1-4, you roll 1d8 as a base, which means you have a 12.5% chance to roll one 8 and get a bonus damage dice. Each 1d8 has an average damage of 4.5, so your average damage is 4.5 + (12.5%)(4.5) = 5.06. That's still less than a Firebolt's average of 5.5, but you do get to change your damage type every turn AND you're more likely to do 10 or more damage than a Firebolt (10% chance of 10 damage, vs (12.5%)(87.5%) = 10.9% chance of rolling an 8 and then at least a 2 to deal 10+ damage). At the same time, Sorcerous Burst is much more likely to do negligible damage than a Firebolt. A 5e goblin has 7 HP, for example. If you hit one with Firebolt, you have a 4/10 = 40% chance to deal at least enough damage to kill it with one shot from full health. If you hit one with Sorcerous Burst, however, you have a 2/8 = 25% chance to deal enough damage to kill them.

From levels 5-9, you roll 2d8, which gives you a 21.88% chance of rolling exactly one 8, and a 1.56% chance of rolling two 8s, for a total average damage of 10.13 (vs Firebolt's 11). At lvl11, the average damage goes to 15.19 (vs 16.5), and at lvl17 it goes to 20.25 (vs 22). So it's the same pattern at every level: Firebolt does more on average, but Sorcerous Burst has better chance to deal high damage, has a much higher potential damage cap, and its damage can be changed if damage type matters...while it also has a greater chance of doing a small amount of damage. It is, in short, a swingy and unpredictable spell, which is very thematic.

I think Sorcerous Burst is the right pick for a sorcerer looking for a damage-dealing cantrip. It's not strictly better than Firebolt, and is more likely both to overkill its targets and to not deal enough damage when you need it to, but it's also more likely to surprise you and deal way more damage than you thought it would. It's also more likely to be useful as a damage-dealer in more situations, since you can change its damage type.