I'd be concerned about planting close to the root system of an old, established yew. Can you give yourself more room by cutting back on the lawn line?

Yaks have pretty buds and trusses, but the plant is not all that attractive in my opinion.
Mike

botann is correct. The yews' root system will extend out to its drip line or further. You can clean out the roots where you want to plant and put in a metal edging to keep the roots out. Or, alternatively you could put in a root barrier and a raised bed. The landscape fabric allows water to pass but keeps roots from passing. Rhododendrons and azaleas have shallow roots so they can live in raised bed on landscape fabric. I would do both, clean out the yew roots and use a raised bed.

I and a lot of other people like yakushimanum, especially the leaves. They have a lot of substance, the indumentum, and rarely have any foliage problems. The downside is that the flowers usually open pink and fade to white, or open reddish and fade to pink. Most yaks are only hardy to -15F. I would recommend hardier forms for you. Some hardy yakushimanums include:

Dr. Lutton (Mrs J G Millais x yakushimanum), hardy to -20F
From the wilds of western Pennsylvania comes this super-hardy, gorgeous plant on a par with 'Trinity.' One of the larger growing yak hybrids, our 20 year old plant is 6 x 6 feet. 'Dr Lutton' has flawless deep green convex leaves with a trace of indumentum. A vigorous and sturdy plant with an upright, compact habit. Flowers in late May, white opening from pink buds, and are especially long-lasting.

Ingrid Mehlquist (Besse Howells x yakushimanum), hardy to -25F
White frilly flowers emerging from lavender buds in small but very full and abundant trusses on a rock-hardy yak hybrid. This low, compact plant has great foliage and buds heavily from a young age. Resistant to root rot disease. Exceptional.

Ken Janeck (yak seedling), hardy to -25F
A good strong fuschine pink in bud, opening to a soft shade of pink and fading over time to white. Indumentum is heavy under the matte, dark green leaves. A compact grower and "good doer", with unusually large foliage for the species yakushimanum. Always reliable in cold climates.

Mist Maiden (an unknown yak hybrid), hardy to -25F
'Mist Maiden' is a more vigorous growing yak with a larger than normal truss, and somewhat wider open flowers. The flowers present the typical yak apple blossom effect, starting out as pink and fading to white. The foliage is dark green with thick tawny indumentum. It is an earlier bloomer than other yaks and seems to be more tolerant of adverse conditions.

Mountain Marriage (yakushimanum x maximum), hardy to -20F
Perfectly globular white trusses sit atop the rosettes of dark green shining foliage like dollops of vanilla ice cream nestled in dark cones. Combining the best characteristics of both parents, toughness and adaptability from maximum and the compact low growing elegance of yakushimanum.

Ruth Davis (yakushimanum x metternichii), hardy to -25F
Red tinged buds open to large pink ageing-to-white flowers for an interesting yak-like tricolor effect. This is a dependable low grower with dark green leaves on a mounded plant. A good yak hybrid possessing year-round attractiveness.

Trinity (Powell Glass x yakushimanum), hardy to -25F
Starting from pink buds, large white flowers open with faint dorsal spotting of light green. Dark green medium size foliage on a well-rounded plant. We believe it to be the best of the many white catawbiense x yakushimanum hybrids. This is one of John Valigorsky's best performers in cold Pittsfield, Massachusetts.

Rarefindnursery's (smirnowii x yakushimanum) Partain Form, hardy to -20F
The flowers all strongly resemble the well-known 'Ken Janeck' and 'Mist Maiden', this plant form and foliage have distinctive differences. This clone has very dark green leaves and is smaller growing and dome shaped with a tan dusting of tomentum on top of the leaf for added interest; and of course, the fuzzy tan indumentum on the leaf underside.

I think I just convinced myself to get some of these this year.

Thanks so much for the advice - I will definitely think about putting in some root barrier. And thanks for the info on cultivars.