Generally You can re-use the conduit. The maximum feed through an existing 3/4" sch. 40 PVC conduit using copper THWN is 3@#6 and a #8 ground being fed by a 70A breaker.
Some Details That Can Matter The NEC limits maximum conduit fill for over 2 wires to 40% of cross sectional dimension conduit. Each wire and conduit type have different dimensions, so actual type of conduit and wire is relevant. There are 80 pages of charts in the code book dedicated to determine the maximum number allowed, but it's easier to use an online calculator.
So using the calculator to determine the maximum wire size you could feed through existing 3/4" PVC using THWN conductors you are looking at a 40A to 70A feed. You put in 3/4", Sch80 PVC, you refer to NEC Table 310.16 75°C column see #6 copper is rated for 65A, so you enter 3@#6 and a #8 THWN ground, and oops, 46% fill, too much. Go back up and change to Sch 40 which has same external dimension, but slightly larger internal dimension, and get 37% fill, you are ok. So the actual type of conduit is relevant. Even with Sch 40 if you're not experienced pulling wire it would be a good idea to consider sticking with a #8 feed. Don't even consider trying to pull UF, USE, or any cable assembly through this (or any) conduit, it requires a different calculation that results in larger conduit being needed.
Buried conduit is considered wet location and the rating of THHN is technically not wet rated, but all the THHN wire I have seen on the market is multirated as THWN also, which is wet location rated.
The actual maximum breaker size is dependent on rating of the terminations, modern panels and circuit breakers are rated for 75°C and are higher rating than 60°C terminations found in old equipment like fuse panels. This rating needs to be confirmed.
#6 THWN is 75°C rated for 65A, which isn't a standard breaker size listed in the NEC, so you are allowed to round up to the next size larger breaker. Although your wire may be THWN-2 no terminations are rated for 90°C, that number is provided for when de-rating is required, which doesn't apply to your question.
Normally local jurisdictional authorities will let you add a 40 to 70A breaker into your service panel without much consideration for what's existing, but when you go to upgrade the service panel they will take into account what you have added to he calculation for service size. Any question on Code issue is also subject to adoption or amendments adopted and interpreted by the local authority.
I didn't consider calculating maximum feed with metal conduit as ground, first because I think it unlikely, and second it's hard to verify the conduit is intact and sufficient for use as the equipment ground. I also didn't calculate using aluminum because I was trying to give you the max available without head spinnninng variables.
As others have noted a 40 to 70A feeder supplying a 100A panel is fine since your sub-panel must be rated for at least the size of the breaker feeding the sub-panel. If you do need to replace a panel bigger is usually only a small increase in price and prevents possible need for upgrade later.
Your statement "The main in the house has 1 15A breaker and nothing else" doesn't really seem like it could mean what is seems to mean.