Without adding an EFI partition and then installing the EFI bootloader(s), you cannot boot in UEFI mode. UEFI mode does not depend upon the disk partitioning type, but usually, UEFI is put on a gpt partitioned disk because Windows requires gpt for UEFI (Ubuntu does not care). If your amchine allows a preference selection for mode (legacy before UEFI, or vice versa), then your machine should still boot (in legacy). If you machine settings only allow UEFI or legacy(CMS), then setting it to UEFI will not boot (no UEFI bootloaders).