Realistic migration timeline + cost for a typical store?

Three rough sizes:

• Small (low customisation, < 10 extensions): 4-6 weeks, $5-10k. Mostly theme port + brand re-skin. Fastest payback.
• Medium (custom Luma child theme, 10-25 extensions, some B2B): 6-9 weeks, $12-20k. Most stores fall here.
• Large (multi-store, heavy custom modules, B2B-heavy, 25+ extensions): 10-16 weeks, $25-60k. Often phased, full Hyvä on B2C storefronts first, B2B portal in a later phase.

Side-by-side QA + soft cutover (10% traffic for 7 days, then full) is included in every estimate. Roll-back plan stays available for 14 days post-cutover.