PAL and SECAM could be converted from one to the other pretty much losslessly with a small (but appropriately expensive) piece of industrial-grade equipment, the difference is just the encoding of color into composite (and broadcasters would internally use RGB anyway).

PAL/SECAM and NTSC were way more tricky as the number of lines (576/625 or 480/525) and refresh rate (25 or 29.97) differed. I think they first had to use film to skip/duplicate every 6th/5th frame and blur the lines; since the 1980s expensive and giant 2MB RAM frame buffers were available with logic that would dispatch missing/duplicate lines and fields accordingly. Today, every phone routinely scales and framebuffers video without batting an eye. Neither method is lossless though.

Czechoslovakia’s public TV switched from SECAM to PAL in circa 1991. Most old color TVs could have conversion kits installed; post-1985 ones like my family’s Tesla Color 462 were already made future-proof by including rudimentary (non-phase-correcting) PAL decoders. Between cca 1995-2010, virtually all TVs had SCART inputs that fully supported RGBA, bidirectional (TV<->VCR) stereo audio and composite PAL, SECAM or NTSC video, as well as control signals such as aspect ratio. (Yes, late 1990s European CRT TVs don’t need RGB or NTSC mods, they have it all! Just the cable and connector is bulky and falls out often because we hadn’t figured out twisted differential pairs in the 1970s.)

Still, TV here is broadcast at 25 f/s and 576i if in SD because of all the old content in that format. I’m furious YouTube never allowed 576i, scaling everything down to 480p with awful combing. But yeah, I’m glad we’ve settled on 1080p for modern content; frame rate conversion is less disruptive than scaling.