Unfortunately not. Serilog will need to make any appropriate changes.
Just in case you're interested in what these two changes are about:
string was replaced by
text back in 5.0 and we did add a "backwards compatibility layer" in at that time and kept it for all of 5.x. There are details about the change here.
_all change is only for new indices in 6.x. So if you have an index that was created in 5.x, you can upgrade to 6.x and it will continue to work as-is.
The second of these may be a workaround for that one if you absolutely need to, but it sounds like Serilog is going to need to make updates regardless of whether the workaround works for one user.
Since I'm also not very familiar with the Serilog project and what it provides/suggests, I'll also mention a few other things that could help with your upgrade experience if they're options. One is that if you install X-Pack in 5.x, it comes with a cluster/index checker which checks if you're using any features that won't work in 6.x. I realize you may have already run into a few stumbling blocks with your upgrade, but I'd recommend using the utility to check anyway since there may be other things you haven't run into.
You should also check the deprecation log for any queries which may need to be upgraded. You can also turn that on with the upgrade assistant if you haven't already.