I want to replace thuja hedge with something else (e.g. amelanchier), but I want to maintain privacy during the process.
What are my options? Can I keep some thujas and growing amelanchier between them and continously swap thujas for amelanchiers?
Or is only possible way to completely dig out thujas, plant new plants and in the meantime use some privacy screen on fence?
It would be better to remove all the Thuja, then dig the area over, incorporating humus rich material such as composted manure, good garden compost, leaf mould and the like, then allow the soil to settle for a week or so before new planting goes in.
I would also just point out that, if you want a privacy screen, it's probably best to put up a hard privacy screen anyway if you want to use Amelanchier, because they are deciduous and tend to have a tree like growth habit, meaning any privacy they eventually give during the summer will be about 6 feet or more above ground level. If you only need privacy from, say, tall buildings nearby, then privacy screening at ground level may not be necessary.
