Conflict between old GRUB2 and newly automatically installed one

I had on acer extensa 2519, 2G RAM, 500G HDD windows7 and ubuntu 18.04 and boot them via grub2 legacy (switchable to efi). As it was slow I installed xubuntu and during installation it strongly requested efi system partition. S I created it and then the installation process failed as there was a conflict when it installed grub2.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: ubiquity 20.04.15.10
ProcVersionSignature: Ubuntu 5.8.0-43.49~20.04.1-generic 5.8.18
Uname: Linux 5.8.0-43-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.445.1
Date: Sun Apr 25 18:33:35 2021
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/xubuntu.seed quiet splash ---
LiveMediaBuild: Xubuntu 20.04.2.0 LTS "Focal Fossa" - Release amd64 (20210209.1)
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)